I have the same problem. Just like others, if I disable Ubuntu Firefox Modifications, it's fine.
-Ubuntu Linux 9.10 Karmic Koala Desktop amd64
-Firefox 3.5.5
-Extensions: Abduction! 3.0.4 Clear Cache Button 0.9b DownloadHelper 4.6.4 DownThemAll! 1.1.7 Firebug 1.4.5 Tab Mix Plus 0.3.8.2 Ubuntu Firefox Modifications 0.8
I have the same problem. Just like others, if I disable Ubuntu Firefox Modifications, it's fine.
-Ubuntu Linux 9.10 Karmic Koala Desktop amd64
-Firefox 3.5.5
-Extensions:
Abduction! 3.0.4
Clear Cache Button 0.9b
DownloadHelper 4.6.4
DownThemAll! 1.1.7
Firebug 1.4.5
Tab Mix Plus 0.3.8.2
Ubuntu Firefox Modifications 0.8