Comment 5 for bug 619151

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 a new profile 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 test a new profile by starting profile manager with the following command, but make sure all Firefox windows are closed first:
firefox -ProfileManager

To disable extensions, Tools > Add-ons > Find the extension >
Highlight and click 'Disable' > Restart Firefox