The best thing would be to download the current and install it again. Firefox; Extended Support Release {web link} ESR Notes System Requirements

Something seems to have gone wrong with the desktop shortcut that starts Firefox, but by looking thru the settings I have no idea what. I copied a different one that still worked and the problem is gone (for now at least).

That was very good work. Well Done.

Do you only have one Firefox version installed or do you possibly have more than one version installed like 32-bit and 64-bit ?

I did. When I just now uninstalled the 64-bit version, the uninstaller ran in German. So it must have somehow gotten switched to that.

I had a similar problem on my Windows 10 computer only a few hours ago. I opened settings, searched for time & language, clicked on language & clicked on: remove German as additional display language for my computer, which in part was understandable via a set of icons. I had originally installed it, thinking, that it would help with my Office 2020 Family subscription in : Word, by automatically correcting correctly also in German. That was not the case, but requires a different path. What installing German in settings as added language accomplished was, that all of my computer display language was changed to German, which is not practical, since I live in the USA. If you have a different OS than Windows, you may want to type into an address bar the internet address for your OS & there ask your question, in case their browser organization uses webpages with different names. It would probably be a synonymous term (word) to settings. The translation into another language or from another language worked that way too for my Windows 10 computer regarding my problem mentioned earlier.