Mozilla yesterday released Firefox version 3.5.6. The update resolves three critical security holes, and also patches up some stability issues.
For full details on the critical fixes, as well as four other security updates rated high, moderate or low, see Mozilla's list of security advisories for Firefox 3.5. For a technical list of all the bugs squashed in 3.5.6, head to Bugzilla.
Clicking Help | Check for Updates will get you the new version, as will the browser's automatic update feature (find its settings by clicking Tools | Options, then select Advanced and the Update tab).