System freeze on intensive left-button scrollbar scrolling

Binary package hint: firefox-3.5

Reproduced several times: keybord/mouse won't work, thus a reboot is the only way out.

On repeated scrolling using the (vertical) scrollbar on various websites, Firefox seems to permanently freeze, except that it's the whole system that freezes.
It seems doing small back and forth motions with the scrollbar (left-button controlled) triggers this very annoying bug.
Maybe it's more probable one hour at least into a Firefox session.

Karmic 64-bit
USB keyboard
PS/2 optical Mouse (USB to PS/2 adapter)

Apparently no flash or Ajax involved: occured on text content intensive websites such as wikipedia where I would need to browse rapidly through documents.

ProblemType: Bug
Architecture: amd64
CheckboxSubmission: 3d3139e272c419276633ca58f4d1fe69
CheckboxSystem: daed2f3d6643b4a84b4520a2427f8c2b
Date: Sat Nov 28 14:26:27 2009
DistroRelease: Ubuntu 9.10
NonfreeKernelModules: nvidia
Package: firefox-3.5 3.5.5+nobinonly-0ubuntu0.9.10.1
ProcEnviron:
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-15.50-generic
SourcePackage: firefox-3.5
Uname: Linux 2.6.31-15-generic x86_64
XsessionErrors:
 (gnome-settings-daemon:1654): GLib-CRITICAL **: g_propagate_error: assertion `src != NULL' failed
 (gnome-settings-daemon:1654): GLib-CRITICAL **: g_propagate_error: assertion `src != NULL' failed
 (polkit-gnome-authentication-agent-1:1770): GLib-CRITICAL **: g_once_init_leave: assertion `initialization_value != 0' failed
 (nautilus:1751): Eel-CRITICAL **: eel_preferences_get_boolean: assertion `preferences_is_initialized ()' failed
 (Do:1769): Wnck-CRITICAL **: wnck_set_client_type got called multiple times.