Mozilla has released the final update for Firefox 3.5, 17th for 3.6 and the first for Firefox 4. These updates fixed several security issues & stability issues.
Firefox 3.5/3.6 users can get the update by going to Help >> Check For Updates. Firefox 4 users go to the Firefox Menu > Help > About Firefox.
Firefox 3.5 users this is the final update. Depending on your operating system you should upgrade to Firefox 3.6 or 4.