hello, this might be an issue with malware on your system. please perform all those steps:

1. reset firefox (this will keep your bookmarks and passwords)
2. afterwards go to the firefox menu ≡ > addons > extensions and in case there are still extensions listed there, disable them.
3. finally run a full scan of your system with different security tools like the free version of malwarebytes and adwcleaner to make sure that adware isn't present in other places of your system as well.

Troubleshoot Firefox issues caused by malware

Thank you,

Malwarebytes has blocked a program detected when opening firefox. Malwarebyters blocks a program datamngr.dll PUP.Optional.Datamngr.A Adwcleaner freezes and firefox crashes when downloading other antivirus.

ok, so there seems to be malware that is actively preventing attempts to remove it.

please restart your computer into windows safemode with networking mode enabled and repeat the steps then: http://windows.microsoft.com/en-us/windows/start-computer-safe-mode#start-computer-safe-mode=windows-vista