Type about:preferences#content<enter> in the address bar.

On the bottom of the page, find Languages. To its right, press the Choose button. Make sure the correct language and district is selected.

Type about:config<enter> in the address bar. If a warning screen comes up, press the Be Careful button. At the top of the screen is a search bar.

Check the spellchecker.dictionary entry.

This is the entry:

services.sync.prefs.sync.spellchecker.dictionary;true

but I don't know how to change it or what exactly to do, so that the language changes into german.

Kind regards, Corinna

Let's go for a re-install.

• 1 Download Firefox For All languages And Systems v43* {web link}
• 2 Uninstall Firefox from your computer {web link}
  DO NOT remove your profiles.
• 3 After, reboot the computer. Then run the full installer.

services.sync.prefs.sync.* prefs set whether to sync the real pref to other connected devices when Sync is used.

• services.sync.prefs.sync.spellchecker.dictionary;true

What could matter is the value of the actual spellchecker.dictionary pref.

Note that Firefox stores the selected language as part of the Site Preferences, so you could try to clear the Site Preferences.

You can delete the content-prefs.sqlite file in the Profile Folder to only remove settings stored in this file. That way you keep exceptions stored in permissions.sqlite that would get lost if you clear the "Site Preferences" via Clear Recent History.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox