Mozilla just released Firefox 9 on 20 Dec 2012, following which a critical tracking bug was found. This bug has forced Mozilla to release the updates on the next day of the release of version 9. You will get this update automatically or you can manually update your Firefox (Help –> About Firefox). Now the Firefox Download page has also been updated with 9.01 version.
There is no major information about the bug, you can read about it in Bugzilla. It seems, it’s a good idea to stay with older version of Firefox for few days than updating it to new version.
Download Firefox 9.01