Some fairly important security updates for Firefox 2.x and 3.x were released yesterday.
See http://www.mozilla.org/security/announce/ for more info on the bug fixes included.
If you still use Firefox 2.x, this release is the last planned upgrade . . .
For users of FireFox 3.x, see this page for news and info:
As always I highly recommend the excellent NoScript plugin for both versions to help make your online browsing experience safer. And remember to check for updates in your Tools:Add-ons menu option every time you upgrade to new builds of Firefox.