Attention Firefox uses, go ahead and restart your browser if you haven't done so recently. Firefox should automatically update itself to version 3.6.12 (assuming it's already downloaded the patch), and while there aren't any fancy new features to speak of, the latest version does fix a critical security issue Mozilla says could potentially allow remote code execution.
"We have received reports from several security research firms that exploit code leveraging this vulnerability has been detected in the wild," Mozilla stated in a blog post. "Thanks to Mozilla's industry-leading open security process the fix has been created, tested, and released to users within 48 hours of first notification about this vulnerability."
The fix also applies to Firefox 3.5 users, which has been updated to 3.5.15. If you haven't received the update automatically, you can force Mozilla's hand by clicking Help > Check for updates.