Firefox 3.5 beta occasionally completely locks system when starting f11 x86_64

[stan]

Hi,

Occasionally, when starting Firefox in F11 x86_64 it will completely
lock the system, requiring a hard reboot.  Is anyone else experiencing
this?  If it is just an idiosyncrasy on my system, I won't bother
opening a problem record.  I have no indication of what is causing
this, it might not even be firefox but something that firefox triggers.
Seems to happen when the tabs from a previous shutdown of Firefox are
loading.

The system is up to date with all the updates from updates-testing so
is running the latest kernel from there.