If you take the Acid2 test in any browser, you will notice that they all fail. This is not because all browsers suddenly stopped passing Acid2, but because of an error with the test itself. …
Ian Hickson's comment in Mozilla's Bugzilla bug tracking system seems to indicate that it will be fixed after the new year, but in the meantime you can try a copy of the working Acid2 test over on Ian's site. The reason for the broken Acid2 test is also covered in the Bugzilla discussion:
It looks like an object element that's pointing to a non-existent page (http://www.webstandards.org/404/) should be falling through, but that page is now returning a response status of 200 (success) instead of 404.
Update 2007-12-22: It looks like this problem has been addressed, so the Acid2 test should be back to normal.