Whenever you get a message / popup like that;

DO NOT USE ANY OF THE PROVIDED LINKS OR ANY PHONE NUMBERS ! !

Such messages are only Spam that could lead you to Fake Support. Not only can they damage or hack your system, they charge you to do so.

You can report such a site at; http://www.google.com/safebrowsing/report_phish/ Google Report Phishing Page which is the same when done while on site by going to Help > Report Web Forgery

http://www.mozilla.org/en-US/legal/fraud-report/ Help us safeguard Mozilla’s trademarks by reporting misuse

https://support.mozilla.org/en-US/kb/avoid-and-report-mozilla-tech-support-scams

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed free to use malware scanners. Each works differently. If one program misses something, another may pick it up.

It definitely sounds like a scam. 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

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.

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.