Comment 2 for bug 618867

Thank you for reporting this to Ubuntu. Sometimes Firefox profiles can become corrupt and/or extensions can conflict. Could you please try this with a new profile and see if the problem still exists? You do not need to delete your old profile to test. If this helps, you might want to try to disable all your extensions and re-enable them one by one until you find the problematic one.

You can start the profile manager with the following command:
firefox -ProfileManager

By the way, this worked for me fine in a clean profile.