Mozilla just released a small security update for Firefox, bringing the 'fox to version 3.5.4. Like most of these updates, you're mostly looking at security issues and a few stability improvements.
You can read through the full changelog here. On the Mac, I was pretty keen to see the "slow script dialog appears while print dialogs are open" fix, since it's an annoyance I'd dealt with. You should automatically be prompted to upgrade sometime in the next day or so, but if you don't feel like waiting, you can also go ahead and grab the latest yourself here.