I am having a problem with firefox in ubuntu... occasionally when I hit the back button, FF greys out, like when you bring up the 'shut down' dialog in windows, or when you are asked for a password in Ubuntu... but never comes back up... I actually have to kill the process for ff, and restart it. Has anyone else run across this? Or have any suggestions? any help is much appreciated.

here are the specs.

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.8) Gecko/20071022 Ubuntu/7.10 (gutsy) Firefox/2.0.0.8