I had a similar issue after upgrading to latest -current (i.e firefox 3.6).
I reinstalled scim-bridge package as suggested by sunny_5252 and the issue disappeared. Unfortunately I can not remember the scim-bridge error but my screen just froze.
I suspect that running the doinst.sh script (see code below) in the scim-bridge package would be sufficient but I can not test it anymore
if [ -x /usr/bin/update-gtk-immodules ]; then