hi, you're on beta or dvedition with a language pack that became incomplete. try starting safe-mode by pressing the shift-key while firefox is launching, open about:addons and check if you have language packs installed. try to update them manually, or remove them, then restart normally and your firefox is hopefully recovered.

philipp said

try starting safe-mode by pressing the shift-key while firefox is launching, open about:addons and check if you have language packs installed. try to update them manually, or remove them, then restart normally and your firefox is hopefully recovered.

thank you very much, it helped a lot!