Did you uninstall previous FF before installing FF German version? On Mozilla site there are different version for different languages on there.

https://www.mozilla.org/en-US/firefox/all/

well The German version was the first ever installed...

https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

Maybe you have both Firefox 54 and 58 installed or Firefox browser useragent is stuck at Firefox 54.

Thank you for your reply...I guess the user agent got stuck at FF 54. What could I do??

Did you look at article I posted?

doh ..missed the Fedora in Firefox 54.0 useragent.

Your user agent shows as a Firefox 54 version from the Fedora repositories.

• User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0

You will probably have to check the repositories for language packs for that version. Otherwise you can consider to switch to the official release.

Did you check the actual version in "Help -> About Firefox" ?

Sorry I didn`t, I will. Thank You. I'll let you know...

Well i did change the user agent and cleared the cache, nothing happened...

"Help; ...about Firefox" says I have FF 58.0...

but if I want to download the current Language Pack it says I cannot do so because my version is 54.0... even though " About Firefox" says I have the latest version (58.0), So the Language Pack I have has been disabled as long as i have not upgraded to 58.0... Crazy, isn it? That's my problem...

Did you look for a general.useragent.override pref on the about:config page?

What does the on the "Help -> Troubleshooting Information" (about:support) page say about the currently installed Firefox version?

You can create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I created a new profile.. And it worked.. It was the old profile... I migrated all my data to the new profile... and.. Can I delete the old profile?? and use the new one as default? Thank you very much!!!