Yesterday my wife updated to Firefox 4 and today we haven't been able to boot up her computer,
it just freezes during start-up then restarts,over and over. Checked on the Firefox/Mozzilla website
and they said there has been problems with system "slow-downs" and multiple crashes reports
by many people. They did not have any fixes or work-arounds available at this time.

Just thought I'd pass that along.

ditto.....doing my swede in!

Are you able to boot into safe-mode and uninstall it?