Comment 59 for bug 281348

Same problem here. Disabling tabmixplus or ubuntu modifications works around it.
Karmic.
ubufox 0.8-0ubuntu1
firefox-3.5 3.5.5+nobinonly-0ubuntu0

Last updated: Sun, 15 Nov 2009 16:58:04 GMT

User Agent: Mozilla/5.0 (X11; U; Linux i686; he-IL; rv:1.9.1.5) Gecko/20091109 Ubuntu/9.10 (karmic) Firefox/3.5.5

*** Extensions (enabled: 24, disabled: 3; total: 27)
Adblock Plus 1.1.1
bit.ly preview 1.266
Boost for Facebook 9.8.6
ColorZilla 2.0.2 [disabled]
Converter 0.8.6
Dictionary Switcher 1.0 [disabled]
DownThemAll! 1.1.7
Firebug 1.4.5 [disabled]
Flagfox 3.3.18
Flash Video Resources Downloader 1.0.2
Flashblock 1.5.11.2
Google Gears 0.5.32.0
Greasemonkey 0.8.20090920.2
gTranslate 0.5.1
hid.im 0.3.2
InfoLister 0.10.1
Launchpad Improvements 0.8
Novell Moonlight 1.99.8
Personas 1.4
Read it Later 0.9948
Screengrab 0.96.2
Show my Password 1.7
Tab Mix Plus 0.3.8.2
Ubuntu Firefox Modifications 0.8
Update Notifier 0.1.5.5
User Agent Switcher 0.7.2
WOT 20091028

*** Themes (1)
Default [selected]

*** Plugins
DivX® Web Player
Java(TM) Plug-in 1.6.0_16-b01
QuickTime Plug-in 7.2.0
Shockwave Flash
Silverlight Plug-In
VLC Multimedia Plug-in
VLC Multimedia Plugin (compatible Totem 2.28.1)
Windows Media Player Plug-in 10 (compatible; Totem)
Xine Plugin