Comment 1 for bug 366668

Many bad Firefox behaviors are in fact caused by one of the many third-party 'themes', 'extensions' and 'plugins'. These will go away with a new profile - that is, when Firefox is restored to default settings and without any extensions and themes. You do not need to delete your old profile to test.
Could you please try with a new profile and let us know the results?
https://wiki.ubuntu.com/MozillaTeam/Bugs?action=show&redirect=DebuggingFirefox#Try%20with%20a%20new%20profile