I work on the Firebug web debugger. We have several reports of problems with wordpress.
http://code.google.com/p/fbug/issues/list?can=2&q=label%3Awordpress
The one that I can reproduce happens when we reload the wp-admin page with Firebug suspended.
The wordpress page is too complex to sort this out directly. So I am hoping that someone from the wordpress team will contact us. Together maybe we can figure it out.
jjb
