Comment 5 for bug 938981

Same problem. Started to happen after the (automatic) upgrade to firefox 10, in my case on February 14.
As the OP, crashes sometimes manifest as Xorg restarts, other times system freezes completely, showing a black rectangle on top left of the screen. It is deterministic: it always the third time I open Firefox (tabs do not count)

firefox is 10.0.2+build1-0ubuntu0.10.04.1
fglrx is 2:8.930-0ubuntu1 (package build from ati installer)
ubuntu is 10.04 (32bits)

ATI Radeon HD5450 graphics card

I use 3D design tools like Blender with no problem.

kern.log is full of entries like:

Mar 6 10:12:53 titan kernel: [ 176.203479] type=1503 audit(1331039573.125:34): operation="open" pid=3013 parent=3011 profile="/usr/lib/firefox-10.0.2/firefox{,*[^s][^h]}" requested_mask="::rw" denied_mask="::rw" fsuid=1000 ouid=0 name="/dev/ati/card0"

and

Mar 6 11:35:17 titan kernel: [ 5120.649544] type=1503 audit(1331044517.573:210): operation="open" pid=3011 parent=1 profile="/usr/lib/firefox-10.0.2/firefox{,*[^s][^h]}" requested_mask="r::" denied_mask="r::" fsuid=1000 ouid=1000 name="/proc/3011/smaps"