TBB does not disable other firefox extensions
When I run the current TBB (tor-browser-gnu-linux-x86_64-1.0.17-dev-en-US.tar.gz) on Ubuntu Maverick system wide Firefox plugins are loaded into the TBB instance.
Most prominently is the "Ubuntu Firefox Modifications" add on; other Firefox properties seem to be enabled - such as DivX, Totem as a video handler, and others.
This is probably very dangerous even with Torbutton unless Mike Perry says otherwise.