This is some problem with Firefox, not Firebug. You can try Firefox
support, but so far no one has reported success with that approach.
Reported successes were from install in a new profile, temporarily
disabling anti-virus, re-installing the OS to remove viruses, and
updating anti-virus program Kapersky. See also
http://getfirebug.com/wiki/index.php/FAQ.
jjb