Has anyone seen a "lock up" occur on FC6 when running Firefox?

[Richard England]

Kernel  2.6.18-1.2869.fc6 smp  fully updated install

firefox 1.5.09 via  yum

If I start firefox and use or let it sit idle, eventually the system 
will lock up.  No keyboard response, cursor moves with mouse movement 
but the buttons do not function.   I can't conclusively say that firefox 
is that cause but it seems to be the commonality.  If I let the system 
idle without firefox, I have yet to see a freeze.

Any one else seen this?  Any one have some diagnostic tricks I can use 
to isolate this?  I'm not a complete novice in using Linux but any tips 
might save me a boat load of trial and error.  I may try starting a VNC 
session from another machine and see if it is active so I can see dmesg, 
etc.  Perhaps it's an xwindows issue.....

Thanks,

~~Richard