Comment 5 for bug 566412

This also happens to me. I assume it is related to the video drivers, though.

My reasoning for this is that
1) I can reproduce this bug not only in Firefox but also in Evince when scrolling through PDF files.
2) I can fix the issue like this:
  a) wait for the system to freeze
  b) press Ctrl+Alt+F1 (which is pretty much the only thing that works when the system is "frozen")
  c) wait about 5 seconds (required!!)
  d) then return to X with Ctrl+Alt+F7
  ==> RESULT: the system resumes working normally

My system:
- Intel Core 2 duo, running 32-bit Ubuntu 9.04
- ATI Mobility Radeon x1300 running the open-source drivers
- Compiz
- Firefox 3.6.3
- Flash non-free 10.1