This is a recent problem but I have no idea what caused it: I can only run firefox in safe mode. Any other mode, I get a completely khaki, unresponsive screen, that I have to forcibly terminate.
I have uninstalled firefox, and then re-installed it via synaptic. That didn't seem to work.
The problem occurs in all the four user accounts I have defined in the ubuntu.
I am running an ubuntu 11.10, amd64. My firefox is version 8.0.