It looks that the problem is related to some leftover files in /usr/lib/firefox/components, especially compreg.dat. Removing compreg.dat resolved this problem for me. Does this work for you as well?
See upstream bug for further details. -- GMail and Ajax websites not working with Firefox 2.0 backend https://launchpad.net/bugs/57888 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
