Mozilla security researcher moz_bug_r_a4 reported that the XMLHttpRequestSpy module in the Firebug add-on was exposing an underlying chrome privilege escalation vulnerability. When the XMLHttpRequestSpy object was created, it would attach various properties of itself to objects defined in web content which were not being properly wrapped to prevent their exposure to chrome privileged objects. This could result in an attacker running arbitrary JavaScript on a victim's machine, though it required the victim to have Firebug installed.
This is now public http://www.mozilla.org/security/announce/2010/mfsa2010-21.html
This issue has been addressed in following products: Red Hat Enterprise Linux 5 Red Hat Enterprise Linux 4 Via RHSA-2010:0332 https://rhn.redhat.com/errata/RHSA-2010-0332.html