Comment 2 for bug 301022

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.
Please follow these instructions and try that site with a new profile:
https://wiki.ubuntu.com/MozillaTeam/Bugs?action=show&redirect=DebuggingFirefox#Try%20with%20a%20new%20profile