Comment 2 for bug 531583

I have no trouble with Greasemonkey in a clean profile. I would suggest checking your scripts to see if one of them is causing the issue:
http://wiki.greasespot.net/FAQ#I_installed_a_script_that_crashes_Firefox._How_can_I_get_rid_of_it.3F

Please let me know if this helps. This seems to be affecting people across multiple distros and someone did open a bug upstream which I'm linking.