Firefox 2.0.0.8 is out today, released by Mozilla. The update fixes several compatibility and security issues, and now works with Mac OS X 10.5 "Leopard."
Some who have installed the browser update also report other problems with some media plugins, Adobe Flash content failing on Intel-based Mac systems, and the 'close other tabs' function failing when more than 20 tabs are open, but these issue are specific to Leopard, not new issues introduced to other platform versions.
The security fixes do plug several holes on Windows, Linux and Apple platforms. The holes fixed by this update were exploitable by common trojans and worms circulating the net currently, so unless you are a power-browser or an Intel-based Mac user, the security update is probably a good idea.