Can you remove the extension in Firefox's Troubleshoot Mode? That should disable both extensions and userChrome.css/userContent.css modifications that might prevent full use of the Add-ons page. The process of restarting in Troubleshoot Mode also often provides access to the Refresh feature, but I think it is worth trying manual removal (Open/Öffnen) first.

Ref. Diagnose Firefox issues using Troubleshoot Mode

If you don't get a German version, try: https://support.mozilla.org/de/kb/probleme-analysieren-fehlerbehebungsmodus-firefox

Also, you probably know that you are running a fairly old version of Firefox with published security vulnerabilities. Is anything in particular discouraging you from updating?

Also, you probably know that you are running a fairly old version of Firefox with published security vulnerabilities. Is anything in particular discouraging you from updating?

No, i didn't know that, i haven't recieved information about a new Version, i alway update when it's offered. I reinstalled firefox with deleting everything left on my PC from it after deinstalling so ig uess that counts as an update. And to your first answer, troubleshoot mode disabled the extension but didn't allow it to be deleted and the german version of the article, which i tried first, didn't help either. It is gone after reinstall and the button is now there. Thanks for your effort.

Thank you for reporting back. It's possible the old program folder contained a policy file that force-installed the unwanted add-on (preventing removal) and blocked updates (and who know what else it did). It's just speculation at this point, but it's probably more important to be in a good place than to know exactly what was going on.

If you start Firefox with a specific profile by adding a parameter (-P profile) to the command-line then you may also not be able to refresh Firefox.