This a rare instance of Microsoft releasing a patch on a day other than its regularly scheduled Patch Tuesdays, and indication of how serious the vulnerability was -- and especially how aggressively the hole was being exploited.
The security bulletin with links to various IE version patches is here.
Patch all machines running IE now.
The patched version of Firefox is 3.0.5; this is the only version you should be running.
Firefox 3.0.5 can be downloaded here.
Patch all machines running Firefox now.