Apple OS X Snow Leopard (10.6) is the latest version of the Apple OS. Firefox 3.0 and 3.5 will work on Snow Leopard the same as they do on Leopard.
After you update to Snow Leopard, rebuild the font caches.
Java content may crash Firefox on Snow Leopard when loading certain pages. To fix this, update to the latest version of Firefox. For instructions, see Update Firefox to the latest version.