hello Steffen47 , [https://support.mozilla.org/en-US/questions/940393#answer-377118 from another user] we found out that this might be related to malware active on your pc. please close all firefox windows and try [[Troubleshoot Firefox issues using Safe Mode|running firefox in safemode]] (by pressing the shift key while you're starting it). in firefox safemode go to firefox > addons > extensions and remove the entry named "Codec v1.0" or any other addons that look suspicious (toolbars etc.) & you have not installed intentionally. afterwards see if you can run firefox normally again... please also do an in-depth scan of your system with the security software already in place and/or a different tool like the [http://www.malwarebytes.org/products/malwarebytes_free free version of malwarebytes] & keep firefox and your plugins up-to-date in order to stay safe on the web. [[Troubleshoot Firefox issues caused by malware]] [[Update Firefox to the latest version]] https://www.mozilla.org/plugincheck/

Is your prefs.js file in your Firefox profile undamaged? I had a similar kind of problem when a line in my prefs.js file got chopped off short. Alternatively, it could be that your localstore.rdf file in the profile is damaged. * http://kb.mozillazine.org/Corrupt_localstore.rdf