If you installed Firefox 3 since its release, you may have noticed that Mozilla issued an update for the browser today. The update wasn't major, fixing a small list of security flaws and bugs discovered since release.
Some of the fixes include problems with Firefox 2 sitting side by side with Firefox 3, security certification warnings, a crash bug on Mac OS X and a potentially more serious flaw in CSS. You can read more in the release notes. Don't forget to update. After all, frequent updating is what earned Firefox 2 the "most secure browser" title earlier this year.