You say you just updated. Did you uninstall the previous installation? Sometimes leftover stuff can cause problems.
Also, a new profile can often fix things. From a command prompt, type:
firefox.exe -Profilemanager
and start with a new profile, and see if the problem goes away. If so, you can transfer the following files to keep most of your configuration:
bookmarks.html
cookies.txt
cookperm.txt
formhistory.dat
history.dat
key3.db
signons.txt
If you don't understand what I'm saying about profiles, follow this link:
texturizer.net