Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Can't even access safe mode; I just tried holding Shift while opening Firefox and it still won't open/start.

Let’s do a full clean re-install;

Note: Firefox comes in three or more folders on all computers. They are;

Maintenance: (Programs Folder) <Windows Only> Firefox itself: (Programs Folder) And two folders in the profile of each user on the computer for each Firefox profile for that user.

If you remove the Firefox folder, the user profiles would not be affected.

Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Note: Check Both Folders

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop

Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove any profile folders.

After rebooting the computer, run a registry scanner if you have one. Then run the installer.

If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

Hi tommyhandley, if the problem continues, Windows might have some log entries about the "crash."

In the Windows 10 search box, if you type event Windows should show the icon for the Event Viewer. Click that, and then find your way to Windows Logs > Application. This can take a long time to load. The most recent events should be at the top. If you find an entry for Firefox, there may be information about conflicting file or application. It's often difficult to track back to a particular program, but worth a try when there are no other clues.

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Booting Windows 10 in safe mode allows Firefox to open and run normally; but I don't know how to fix the problem now, as it just randomly stopped working.

As a test, Disable your protection programs.

Disabled my Avast, have no other antiviruses, still doesn't work.

Then something else is blocking Firefox.

Hi tommyhandley, were there any relevant events in the Applications log?

Hi tommyhandley, if the problem continues, Windows might have some log entries about the "crash." In the Windows 10 search box, if you type event Windows should show the icon for the Event Viewer. Click that, and then find your way to Windows Logs > Application. This can take a long time to load. The most recent events should be at the top. If you find an entry for Firefox, there may be information about conflicting file or application. It's often difficult to track back to a particular program, but worth a try when there are no other clues.

Nope, nothing in the Event Viewer.

Did you try to reboot the computer, possibly in Windows Safe Mode with network support ?