You may very likely need to a file a bug here
However, just to be safe ensure you are on the most recent version of Firefox (10) and verify whether the problems occurs with and without compiz (unity) window manager. For instance, does the issue persists after running
Code:
sudo metacity --replace &
edit:
I am sorry, I see now that you are not running compiz and you specified this. Can you tell us what version of Gtk you have installed?
In terms of a practical fix, try installing firefox-trunk builds; see here
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