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Firefox doesn't start despite not being already running task manager process

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I have recently updated my Firefox version to the latest 3.6.3. I had the feeling the installer donload was not quite "complete", and after installing and running the installer I tryed adding plugins but the latest ones said I have version "3.0.5 MEGAUPLOAD" (I think). I assumed that was caused by the download not really finishing (appears as done after 97% was downloaded), so I redownloaded it and made sure it got up all the way to 100%, using a different download manager. It worked, and I installed firefox again with the completely downloaded installer. When I tried opening it nothing happened, and I checked the task manager but no firefox process was running.

I have recently updated my Firefox version to the latest 3.6.3. I had the feeling the installer donload was not quite "complete", and after installing and running the installer I tryed adding plugins but the latest ones said I have version "3.0.5 MEGAUPLOAD" (I think). I assumed that was caused by the download not really finishing (appears as done after 97% was downloaded), so I redownloaded it and made sure it got up all the way to 100%, using a different download manager. It worked, and I installed firefox again with the completely downloaded installer. When I tried opening it nothing happened, and I checked the task manager but no firefox process was running.

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Does it work if you start in Firefox Safe Mode? See Diagnose Firefox issues using Troubleshoot Mode

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Check the general.useragent prefs on the about:config page. You can open the about:config page via the location bar, just like you open a website. Filter: general.useragent If any general.useragent prefs are bold (user set) then right-click that pref and choose Reset.

See Find what version of Firefox you are using and Web sites or add-ons incorrectly report incompatible browser See also http://kb.mozillazine.org/Resetting_your_useragent_string_to_its_compiled-in_default