Firefox Support troubleshooting guide

Contributors, Firefox Contributors, Firefox Last updated: 01/06/2024 66% of users voted this helpful

This page documents common troubleshooting techniques that are used to solve some of the issues frequently asked in the Firefox Support forum. Most information needed to help users is documented in knowledge base articles. With any problem, try to find an answer before falling back to troubleshooting.

As a helper on the forum, you may encounter problems that you can't find information on. This page can help you find the cause of a problem, as well as help you fix problems after determining possible causes. Information learned while troubleshooting should be shared with others. You can post anything you learn in a related thread in the contributor forums or discuss it in the SUMO Matrix room. Some common support issues don't have documentation yet and need more information.

Common troubleshooting steps

With any issue, always search the knowledge base before attempting to solve the problem on your own. If an issue is not documented, or you aren't sure what the issue is, using the troubleshooting steps on this page may help you narrow down the problem.

Verify everything

It's important for you to understand exactly what a problem is before offering advice. At the same time, it's important for the user to explain all symptoms and other problem details.

  • Ask vital questions such as: What exactly happens? What do you expect to happen? How often does it happen? What are you doing when it happens? Does it affect all websites? Did you install or update any other software?
  • Get a screenshot: If you aren't sure what's happening based on the user's description, ask for a screenshot.
  • Check the installed version of Firefox, the operating system, installed add-ons and other details that may help you find the source of the problem. Always view the “More system details” section of the support question for additional troubleshooting information. If the support question doesn't include such information, ask the user to submit it – see Use the Troubleshooting Information page to help fix Firefox issues for details.
  • Verify details of other installed software, such as the exact names/versions of any antivirus software, etc.
  • If there is an error, verify the exact text. Many error messages can be confusing, so verifying the exact text (or as much as the user can remember) helps with correctly identifying a problem. It's important to know the difference between an operating system error, a Firefox error page, and a javascript "alert()".

Verify exact behavior for lost or missing data issues (bookmarks, history, settings, cookies)

While verifying symptoms is important for all issues, it is especially important for issues involving data loss. Such issues include data being lost, data not being saved when Firefox is closed, and data unable to be changed.

  • It is useful to know where Firefox stores data, as corrupt or locked files often cause data storage issues.
  • Verify as many details as possible. Ask what is happening and what the user expects to happen.
    • Do data changes (new bookmarks, history items, back/forward button history, setting changes, new cookies) show up at all? Does an error result?
    • Does new data appear after closing and restarting Firefox?
    • Has the data loss occurred more than one time? When did/does it happen?
    • Are only some uses of data affected (such as changing a certain bookmark or saving cookies on certain sites), or are all uses of the data broken?
    • Which extensions are installed?

Problems on an individual website

If a certain website is misbehaving or producing error messages, an extension (such as Adblock Plus), bad items stored in cache, or a bad cookie could be causing the problem.

  • First try clearing the browser cache, as problems can be caused when files are downloaded wrong or when outdated versions are stored.
  • If the problem still occurs, check installed extensions. If you suspect that one of the extensions could be causing the problem, start Firefox in Troubleshoot Mode.
  • If the problem still occurs, a bad cookie could be causing it. In this case, the user needs to delete cookies originating from the broken website.

Check Add-Ons and try Troubleshoot Mode

Extensions are the cause of many issues relating to broken websites or Firefox not working properly. If a user has extensions, we often ask the user to try reproducing the behavior with extensions disabled or in Troubleshoot Mode. For more information, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Note: Besides disabling add-ons (extensions and themes), Troubleshoot Mode also turns off hardware acceleration and WebGL, disables the JavaScript Just-in-time (JIT) compiler and ignores userContent.css and userChrome.css files (if present). The xulstore.json file, which saves toolbar and window customizations, is also ignored; however, changes stored in modified preferences remain in effect.

Reset preferences

If the reported problem still occurs in Troubleshoot Mode and the user's troubleshooting information shows a large number of modified preferences, consider asking the user to reset individual preferences using about:config (select each modified preference and click the “Reset” Fx71aboutconfig-ResetButton button) or to reset all preferences to the default values (refer to Reset Firefox preferences to troubleshoot and fix problems for details). Alternately, ask the user to try a new Firefox profile. Another option would always be to Refresh Firefox, but this will remove all extensions, added themes and other user customizations.

Reset the xulstore.json file

The xulstore.json file in the Firefox profile folder stores information about toolbars, window positioning, and other interface elements. If changes to toolbars and window sizes are not saved, toolbar icons are missing, or if there are strange problems with the browser interface, resetting this file can help.

Using the Refresh Firefox feature will reset the xulstore.json file to its default settings, but this will remove other user customizations such as extensions, themes, and modified preferences. You can also reset the file by removing it from the profile folder manually.

Get crash reports

If the Mozilla Crash Reporter triggers and the user submits the report, looking up the crash ID is useful for solving the problem. For startup crashes, it is necessary to get the crash IDs manually. If Firefox runs, crashes can be viewed at about:crashes. See the Mozilla Crash Reporter article for details on obtaining crash reports.

Check security software

Some issues are known to be caused by various firewall and antivirus software, especially issues with connecting to websites. If such an issue is reported, it's important to:

  • Ask for the exact name of all security software products installed.
  • Ask which version of each product is installed.
  • Find out when each product was updated and if it has expired.
    • Expired security software causing problems should be removed since it can't be updated.
  • All installed Firefox extensions – many security products install them.
  • Ask whether any security software has been uninstalled. Often, parts of uninstalled security software are left behind unnoticed. If security software needs to be removed, uninstalling from Add/Remove Programs in Windows is always the safest method. If there are problems uninstalling, some companies offer a removal tool.

Clean installation

Some problems with Firefox are caused by a corrupt installation folder, by old files in the location folder, or by extra installed components. Performing a clean installation is a quick way to rule out these factors. All user data is stored in the profile folder, so performing a clean installation will not cause lost data.

  1. Close Firefox completely (File > Exit on Windows and Linux, Firefox > Quit on Mac).
  2. Rename the Firefox installation folder or move it elsewhere.
    • Windows: This is usually C:\Program Files\Mozilla Firefox for 32-bit systems and C:\Program Files (x86)\Mozilla Firefox for 64-bit systems. “Mozilla Firefox” can be renamed to “Old Firefox”
    • Mac: This is usually Macintosh HD/Applications/ Macintosh HD/Applications can be opened in Finder, and Firefox can be dragged to Trash.
    • Linux: The install location varies based on the distribution and installation method. Linux distributions usually don't put Firefox in its own folder, so this method can't be used.
  3. Download Firefox again from and install according to the installation instructions for Windows, Mac, or Linux.
  4. If this fixes the problem, you can optionally check the old install folder for global extensions, components, or corrupt/locked files.

Secondary methods

If you can't find a knowledge base article or other resource to solve a problem and the basic troubleshooting methods don't solve it, try these methods:

Try a new Firefox profile

If you don't want to suggest the Refresh Firefox feature (for example, the user has many customized settings or installed extensions he doesn't want removed) consider suggesting a new Firefox profile. Firefox's Profile Manager is used to create and modify Firefox profiles, each of which has unique bookmarks, preferences, add-ons, and data (see where Firefox stores data).

  • The Profile Manager runs early in the start-up process, so it is useful to test when Firefox will not start.
  • It is often useful to create a new profile for testing, to see if a problem is being caused by something in the user profile.
  • If a new profile doesn't resolve the issue, the user can easily return to his original profile by running the Profile Manager again.

Clear macOS system caches

Some issues with Firefox can be caused by bad data in the macOS system caches. These include:

  • Firefox will not install due to a DMG mount error
  • Weird graphics issues in Mozilla applications
  • Certain font glyphs are messed up or wrong in Mozilla applications

Instructions to clear the caches:

  1. Start the Mac in Safe Mode:
    • Be sure the Mac is shut down, then press the power button and hold down the SHIFT key right after the Mac startup chime. See Apple's support article How to use safe mode on your Mac for details.
  2. Restart the Mac normally.

Suggest anti-spyware or antivirus

If a problem with Firefox is being caused by malware, suggesting antivirus or anti-spyware software can be helpful.

  • It is best to advise users of free-to-use software, rather than recommending specific software to purchase.
  • Mozilla doesn't recommend or endorse any specific products. Free-to-use products popular with the community are listed in the Troubleshoot Firefox issues caused by malware article.
  • It is up to the individual helper to recommend specific software. Make sure that the user knows that software you recommend is your personal recommendation, not Mozilla's.

Obtain a list of modules in the Firefox process

Obtaining a list of modules in the Firefox process can help identify several problems, usually on Windows.

There are two primary methods to obtain the list of processes:

  • Method 1: Use ListDLLs (Windows only)
    • ListDLLs is a Sysinternals application that can list running modules in any process without restarting it.
    1. Download ListDLLs from
    2. Extract listdlls.exe from; put listdlls.exe on the Desktop.
    3. Open the Run dialog (Go to StartRun or hold down the Windows key and press R).
    4. Type cmd and press OK.
    5. You will get to a command line. Type: cd Desktop
    6. Press Enter/Return; the prompt will go to the next line.
      Type: listdlls firefox.exe > mozillabug.txt
    7. Press Enter/Return again. A file called “mozillabug.txt” will be created on the Desktop. This file should be posted onto a site like PasteBin for the volunteers to read your text file.
  • Method 2: Generate a crash report (all operating systems)

An easy way to generate crash reports is by opening about:crashcontent to crash the tab content. This won't open the crash reporter UI, though. Ref: Bug 1490061

  1. Visit about:crashcontent to generate a content crash.
  2. Visit about:crashes and submit the crash report.
  3. Use about:crashes to obtain the newly created crash report.

Check for running Firefox processes

Some issues, especially those related to Firefox not starting properly, can be caused by an existing Firefox process, such as when Firefox hangs. When Firefox won't start, an error says it's already running, or if you suspect that files could be locked, checking for running processes is a good idea.

  1. Check the process list to see if Firefox is running, and if so how many instances are running. The procedure and name of the binary vary by operating system.
    • Windows (firefox.exe): Open Task Manager by pressing Ctrl + Shift + Esc, switch to the Processes tab, look through the entire list for all firefox.exe processes.
    • Mac (firefox-bin): Open Activity Monitor from Macintosh HD/Applications/Utilities. Look through the entire list for all firefox-bin process names.
    • Linux (firefox or firefox-bin): With most Gnome-based distributions, select System > Administration > System Monitor. Then select the Processes tab, and look for all firefox-bin and/or firefox processes.
  2. You may need to end processes, especially if there is more than one Firefox process or if Firefox is not starting.
    • Windows: Click on the process name to kill, then click End Process.
    • Mac: Click on the process name to kill, then click Quit Process.
    • Linux: Right-click on the process name to kill, then choose Kill Process
Note: If a problem is being caused by Firefox hanging, killing all Firefox processes may be necessary after every restart. In these cases, it is necessary to restart Firefox twice after killing all processes to verify that a problem is fixed.

Good things to know

When providing support for users, there are many things that are useful to know.

Finding the profile folder

  • Instructions for finding the profile folder are provided in the Knowledge Base. For instructions on a different operating system, simply choose another operating system on the left side of the page under “Editing tools”.

Where Firefox stores data

When troubleshooting, it is often important to know where Firefox's data is stored. It is important that we never cause unintended data loss – this includes cookies! For example, these are some of the important files that are stored in the Firefox profile folder (see Profiles - Where Firefox stores your bookmarks, passwords and other user data for details).

  • places.sqlite – Bookmarks and history, this can become corrupt in some cases.
  • cookies.sqlite – All saved cookies, this can become corrupt in some cases.
  • prefs.js – This file stores all user preferences, including: the home page, activated theme, cookie behavior, clear all history options, extension settings, and printer settings. If this file is corrupt, it can be reset to restore all preferences to their defaults. See Reset Firefox preferences to troubleshoot and fix problems for details.
  • handlers.json – Content types and download actions are configured in the General panel under the Applications section in Firefox Settings (see Manage file types and download actions in Firefox for details). If the handlers.json file is removed, all customized settings for filetypes and protocols will be reset to defaults. Since at least Firefox 98, however, the mailto Content Type entry will also be removed, so a Firefox Refresh is a better option.
  • extensions.ini, extensions.sqlite, extensions.sqlite-journal extensions.json (if found) – Temporary files for installed extensions. These may become corrupt and need to be deleted.
  • logins.json and key4.db – These files store saved passwords and exceptions. Both the signons file and the key file must be copied to back these up.
  • cert9.db – Saved security certificates.

Differences between operating systems

The support forum gets users who need help with Firefox on all three operating systems, along with its mobile platform (Android). Supporting multiple operating systems is not hard, as long as you are familiar with a few key differences between operating systems.

Windows Mac Linux
On 32-bit Windows, Firefox is usually installed in C:\Program Files\Mozilla Firefox. On 64-bit Windows, 32-bit Firefox is in C:\Program Files (x86)\Mozilla Firefox and 64-bit Firefox is in C:\Program Files\Mozilla Firefox. Firefox installed in /Applications folder Installation path varies; most distributions don't have one
Files on the hard drive accessed with Windows Explorer, launched by opening My Computer Files on the hard drive accessed with Finder, launched by clicking Macintosh HD Files accessed through Nautilus or Konqueror, usually from the desktop{PATH}
Right-click used to access context menus On Macs without a right mouse button, Command + Click used to access context menus Right-click used to access context menus
Menu button Fx89menuButton on each window Menu button Fx89menuButton on each window or the global Menu bar at the top of the screen (used for all applications) Menu button Fx89menuButton on each window
Settings: menu button Fx89menuButton -> Settings Preferences: Firefox Menu bar -> Preferences (or menu button Fx89menuButton -> Settings) Settings: menu button Fx89menuButton -> Settings
To quit Firefox, close all Firefox windows or use Fx89menuButtonExit Closing all Firefox windows will not quit the application. Use FirefoxQuit Firefox or use Fx89menuButtonQuit Firefox To quit Firefox, close all Firefox windows or use Fx89menuButtonExit
Running processes available through Task Manager, Ctrl + Shift + Esc Running processes available through Activity Monitor, available in /Applications Running processes available by running ps -e in a Terminal window
The latest Firefox version currently requires Windows 10 or higher (see Firefox users on Windows 7, 8 and 8.1 moving to Extended Support Release). Firefox currently requires macOS 10.15 (Catalina) or higher (see Firefox users on macOS 10.12, 10.13 and 10.14 moving to Extended Support Release). Firefox requires GTK 3.14 or higher (see Firefox System Requirements).

Other differences include finding the profile folder.

Using about:config

about:config is a user interface (UI) for editing advanced Firefox preferences, which is often necessary to fix problems when they are set wrong.

Warning: about:config can be a nifty tool, but it can also be dangerous and render Firefox inoperable. Be careful when editing about:config.
  • The screencast below shows how to use about:config to fix User Agent problems.

Note: The above screencast shows an old about:config UI. See the Configuration Editor for Firefox article for current instructions.

How to take screenshots on all three operating systems

Obtaining a screenshot is helpful when you aren't sure what a user's problem is. A screenshot can allow you to quickly see what a user is referring to.

  • Windows:
    1. Press the Print Screen button on the keyboard.
    2. Open an image editor such as Microsoft Paint. (Microsoft Paint can be found in the Start Menu under All Programs > Accessories).
    3. From the Edit menu, select Paste.
    4. Save the resulting image as a PNG file, then upload it to the forum.
  • Mac:
    1. Press Command + Shift + 3 to save a screenshot in PNG format on the desktop.
    2. Upload the file from the Desktop to the forum.
  • Linux:
    1. Press the Print Screen button to save a screenshot in PNG format.
    2. Upload the file from the Desktop to the forum.

See How do I create a screenshot of my problem? for more information.

Know how to use features that commonly confuse users

As helpers, we need to be exceptionally familiar with areas of Firefox that users get stuck on. Many helpers do not use all of Firefox's features themselves, so a quick refresh of some of the basics is important.

  • Bookmarks and the Location Bar (formerly called the “Awesome Bar”): To learn more, read Bookmarks in Firefox and Address bar autocomplete suggestions in Firefox.
  • Startup options: When Firefox starts up, you can see your home page, a blank page, or your windows and tabs from your previous session. This is configured in the Home panel of Firefox Settings. See Restore previous session - Configure when Firefox shows your most recent tabs and windows for more information on the Session Restore feature.
  • Download locations: The default download location is configured in the General panel in Firefox Settings. In Windows, downloads are saved to the Downloads folder by default.
  • Application management: File type associations are configured with the Applications section of the General panel in Firefox Settings. This includes whether to save files, open with a specific application, or to ask the user each time.

Removing files from the profile folder

If files in the profile folder are corrupt, they can be removed (Firefox will regenerate the files, as needed).

To avoid data loss, always rename files and folders instead of deleting.
  1. Open the Profile Folder. See Differences between operating systems above for specific instructions.
  2. Make sure Firefox is closed.
  3. Right-click (Mac: Command+click) on the file to remove, and select Rename.
  4. Type a new name for the file. A common practice is to simply add .old or .bak to the end of the filename.
  5. Restart Firefox to allow the file to be regenerated.

How to know when a problem is not caused by Firefox

Many questions users ask are actually related to other programs or the operating system. It is important to be as polite as possible and verify information before informing the user that another program is at fault.

  • Have the user try in another browser. If the same symptoms happen in both browsers, the problem is most likely not Firefox. Check firewall software, spyware, and network connections.
  • Check to see whether malware could be causing the problem. See Troubleshoot Firefox issues caused by malware for details.

Where to get help

  • You can use the Support Forum Contributors Advanced Troubleshooting forum to discuss difficult questions and to see if other contributors are able to help.
  • To learn about resources available to support volunteers, read the Contributor News & Resources.
  • Search the knowledge base – Most questions can be fully answered with a quick KB search.
  • For contributors chat, join the SUMO Matrix room.
  • If you have suggestions or feedback about support, or if you think you have encountered a new issue, you can post in the SUMO community discussions forum.
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