There are two Critical issues fixed:
Mozilla Foundation Security Advisory 2008-34
and
Mozilla Foundation Security Advisory 2008-35.

Download link:
http://mozilla.com/firefox/all-older.html

MFSA 2008-35 affects to Firefox 3.0 too, but FF3.0.1 is just coming in the near future.

[moved from "OT: Firefox (FF) 3.0.1 is out"]

A security update is available now (fixing a critical issue) at
http://www.mozilla.com/en-US/firefox/all.html

Related to FF3.0 'MFSA 2008-35' is not the only issue affecting to FF3.0, there is a Critical MFSA 2008-36 released at
http://www.mozilla.org/security/known-vulnerabilities/firefox30.html#firefox3.0.1

'Crash with malformed GIF file on Mac OS X'
http://www.mozilla.org/security/announce/2008/mfsa2008-36.html

FF2.x series doesn't include this vulnerability.