Do you get this in the Firefox safemode ? http://support.mozilla.com/en-US/kb/Safe+Mode and/or a new profile : http://support.mozilla.com/en-US/kb/Managing+profiles
This is a mass search for bugs which are in the Firefox General component, are UNCO, have not been changed for 500 days and have an unspecified version. Reporter, can you please update to Firefox 3.6.10 or later, create a fresh profile, http://support.mozilla.com/en-US/kb/managing+profiles, and test again. If you still see the issue, please update this bug. If the issue is gone, please set the status to RESOLVED > WORKSFORME.
No reply from reporter, INCOMPLETE. Please retest with Firefox 3.6.12 or later and a new profile (http://support.mozilla.com/kb/Managing+profiles). If you continue to see this issue with the newest firefox and a new profile, then please comment on this bug.