Mozilla has released Firefox version 3.5.6, which includes 62 fixes, three of which are critical security updates.
Firefox is set to automatically download updates and prompt you to install them, but I know that some colleagues of mine have reported instances of the auto-update failing, probably due to system security settings. If you're a Firefox user and you aren't prompted to update your browser today, go to the Help menu and click "Check for Updates" to download the latest version manually.
In addition to the security updates, Mozilla has provided patches to common functional problems, such as with Firefox's full screen mode. For the full list of patches in the 3.5.6 update, click here.