I found this in a bit of Javascript used by a banking site today:

// Ultimate client-side JavaScript client sniff. Version 3.03
// (C) Netscape Communications 1999-2001.
// Permission granted to reuse and distribute.

Obviously the “Ultimate client-side JavaScript client sniffer” was written 3 or 4 years ago, and still works flawlessly. I bet it handles everything from Safari to Firefox without a hitch!