You were correct in identifying a scam. Mozilla (and most other technology companies) will not give you contact information on the screen in the form of a popup if you have an issue on your computer. In fact, telephone support is not something that Mozilla even offers.

The first thing I'd recommend is closing Firefox completely (if you have not done so already). Some malicious popups are designed to make the screen appear frozen and can make it very difficult to close the browser. Try repeatedly pressing Alt + F4 on your keyboard if you cannot close Firefox the regular way. If all else fails, you can hard reboot your computer by holding down the power button on physically disconnecting the device from the power.

Next, I'd strongly recommend scanning your computer for malware. Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• Anti-Rootkit Utility - TDSSKiller (for more info, see this alternate TDSSKiller download page)
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• Hitman Pro
• ESET Online Scanner

Personally, I've found that Malwarebtyes and AdwCleaner are most effective at removing most common threats.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Sometimes regular virus scanners (like McAfee) can miss some things. Using an additional scanner can often catch the things that other programs have missed.

Hope this helps.

If it's like the fake FBI/Interpol browser lock page, A way out of a trap page is to (force it if needed) Close Firefox. Wait about 20 seconds for Firefox to finish closing. Right-click on the Firefox icon and select Open New Window. +++++++++++++++++++++++++++++++++++++++++ Sometimes this works. If it opens, close it by Menu > File > Exit. This will flush out that web page from the Session Store.

If FredMcD's trick doesn't work for you:

Unfortunately, once you have loaded an attack page, Firefox's crash recovery feature may keep bringing it back. There isn't a quick and convenient workaround for that, but I can describe a couple not-so-quick options.

Option #1: Hide the Session History Files

This prevents Firefox from restoring your previous session. If you don't care about those open tabs, this is easier. Also, I have a tool to generate a list of links from the old file for reference so you can re-open pages when needed.

Option #2: Change the Crash Recovery Behavior

If this seems to happen to you often, Firefox can be set to show a list of windows and tabs to select from instead of restoring automatically. Then you can pick out what you want and avoid the rest. This is easy to set up when Firefox is running normally, but takes more effort in the situation where you cannot start Firefox.

First Step: Open Your Profile Folder

[Windows] Open Firefox's Profiles folders by pasting this shortcut to the Windows 10 system search box:

%APPDATA%\Mozilla\Firefox\Profiles

That should launch a File Explorer into the hidden path:

C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles

Here you may find just one folder or you might find multiple folders all with names like "ab35cd21.default-release". Find the folder with the most recently modified files.

[Other OS] Check the section of the following article about accessing your profile without opening Firefox: Profiles - Where Firefox stores your bookmarks, passwords and other user data

Hiding Session History Files

Your profile folder should have a sub-folder named sessionstore-backups containing various files from your last session. Right-click that folder, click Rename, and add the word OLD so the folder name is sessionstore-backupsOLD then click away or press Enter to complete the name change.

Before leaving your profile folder, look for any file named sessionstore.jsonlz4 -- normally this file only exists after a normal shutdown, not after a crash or forced termination. If you do find that file, right-click > Rename to sessionstoreOLD.jsonlz4.

When you start Firefox, it should not find any tabs to restore and should start a new session.

If you want a list of what was in a session history file, you can load it into my tool here and then click Scrounge URLs:

https://www.jeffersonscher.com/ffu/scrounger.html

The script should complete in 15 seconds or less. Longer than that and it probably is hanging (tab crash). You can close the tab and try again in a new tab.

If you get a useful list, click the Save List button so you can load the page from your computer when needed.

Modifying Crash Recovery Behavior

When Firefox is running, you can do it this way:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste sess and pause while the list is filtered

(3) Double-click the browser.sessionstore.max_resumed_crashes preference to display an editing field, and change the value to 0 then press Enter or click the blue check mark button to save the change.

But if Firefox is not running, you can edit the prefs.js file which collects your modified preferences. Firefox is rather particular about the format of this file, so you need to use an editor that won't change the formatting.

Don't double-click .js files because Windows will execute them as system scripts. prefs.js won't harm your system, but you never know, so it's good to avoid double-clicking them.

First, you may need to set Windows to show all file extensions rather than hiding common ones. This article has the steps: https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

Next, right-click prefs.js and choose Edit (to open it in Notepad) or Open With (to use a fancier editor).

At the end of the file, on a new line, paste in

user_pref("browser.sessionstore.max_resumed_crashes", 0);

and save the file. That should be equivalent to changing the setting through about:config.

Hopefully one of these tips will work for you.