I have the same problem as the OP. Worrying to find my problem on a thread that's not resolved...
For the record, it's an old PC with an old Nvidia card (GeForce MX 400, or similar); wiped the HD and made fresh install of 11.10. All seems fine except launching Firefox causes a system freeze. Haven't tried Thunderbird yet. Launching from a terminal spews several repeats of this message:
(firefox:2248): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",
I have tried the trick of renaming .mozilla before launching, but that didn't work for me. I did try installing Chromium - runs fine.
This shouldn't be this difficult - and I bet we aren't the only ones with the problem. Any ideas/advice most welcome.