How are you determining that Firefox is running in the background? For example, are you finding Firefox (firefox.exe) in the Windows 10 Task Manager (Ctrl+Shift+Esc) on the Processes tab or the Details tab?

If you confirm firefox.exe running in the Task Manager, you can right-click > End Process on it to "crash" it so you can start Firefox normally. Does that work?

I wonder whether you also are missing the usual shortcuts on the Desktop and Start menu. Is that making it more difficult to run Firefox?

Firefox doesn't have a built-in behavior to start up with the OS. Perhaps another program is doing it, or a Scheduled Task? An useful program to inspect what happens at start up is this Microsoft utility: https://docs.microsoft.com/en-us/sysinternals/downloads/autoruns

It has a lot of tabs in its interface, but I suggest focusing on Logon, Explorer, and Scheduled Tasks.

Yes, I know that Firefox runs in the background by looking in the task manager (processes tab). (At first I just noticed that nothing seemed to happen when trying to start Firefox.)

I have already tried several times to close the Firefox process and then start again but it always runs in the background. I have also tried to stop all apps from running in the background (in the Settings menu) but it doesn't help either, Firefox runs still in the background.

I normally use the shortcut on the Desktop to start Firefox, but I have tried all other ways I can think of to start it, but it doesn't help.

Microsoft doesn't seem able to help, or know how to handle apps.

They just say I must uninstall &reinstall Firefox.

Last time I did this I lost all my settings, adds-on etc. Is there a way to avoid at least this?

LarsGardneus said

I have already tried several times to close the Firefox process and then start again but it always runs in the background. I have also tried to stop all apps from running in the background (in the Settings menu) but it doesn't help either, Firefox runs still in the background.

Do you mean it restarts by itself? Or do you mean you end the process, it's gone, then when you try to start Firefox, instead of getting an application window, Firefox seems to stall part of the way through it's startup with one firefox.exe process running?

I mean the latter. I end the process. Check that it is gone from the task manager. Try to start Firefox again (by clicking the shortcut on the desktop). Nothing seems to happen but when I check the task manager Firefox is back among the background processes (using a lot of CPU).

Exactly what happens I don't know, if it stalls or not, but there never appears any app window. And I do know that when it functions properly Firefox is among the ordinary apps, not the background apps.

And Microsoft appears not to know how to get it back where it belongs.

I don't know about the distinction between background and foreground apps. Background might be a transitory stage until Firefox displays a window. ??

What if you paste this into the Windows search box and press Enter to load it. Does Firefox finally show a window or does it remains hidden?

firefox.exe -new-window "about:home"

I really don't know the difference either. And I don't know what decides if an app is run in the back- or foreground. The windows apps you can move back and forth. (Microsoft doesn't seem to know either)

I copied that command and ran it. Firefox remains hidden.

I gave up and reinstalled Firefox.

I did this carefully and noted that if I tried to keep my adds-on and preferences during the reinstallation the error still remained. If I however left them out, it went fine. But that of course meant that I had to start from zero again and install adds-on and all preferences.

My conclusion is this: It seems reasonable to assume that it was nothing wrong with Firefox per se, but that the files containing the adds-on and preferences info were corrupt and caused the program to run wild.

I don't know if this can help you to understand the problem.

My question is now: is it possible to identify those files that contained the preferences etc? Are they readable?

I want to find a way to avoid to start from zero again if this happens again.

Firefox stores preferences in multiple different files; this article gives details on many of Firefox's settings files: Recovering important data from an old profile.

For future reference, you could try creating a new profile to test whether settings are causing a problem. More info in this article: Profile Manager - Create, remove or switch Firefox profiles.

(To avoid accidents, do not remove profiles without backing up your real data first. See: Back up and restore information in Firefox profiles.)