Hi there,
I was on WPMU 2.7.1 when I needed to ugprade to 2.9.2 due to the need to use the functionality of a particular plugin.
However, after upgrading automatically or manually updating by replacing the files via FTP, I'm faced with this display problem in Firefox, and it seems to be only happening with Firefox. http://twitpic.com/1pda7y/full
I then was advised by andrea_r to follow the steps listed in http://codex.wordpress.org/Upgrading_WPMU, which I did but the problem remained.
I've vaguely remembered seeing someone else report this problem before too, and iirc that person fixed it up manually running the update, which was of no avail to me. Disabling all the plugins I got, or renaming both the plugins and mu-plugins folder didn't do the trick as well.
The list of plugins I have are as follows :-
nStatistics (this is the new one that I need)
PRO Player
Viper's Video Quicktags
All plugins have been upgraded to the latest version as well.
Thanks for any help rendered, this has been a really frustrating time for me :)