FireFox released their version 4.0.1 update today, fixing several security and stability issues. Its always recommended that you keep your software updated, and that's particularly true for browsers which are your gateway to the online world.

If auto-update is not enabled, then update by clicking Help -> About. The About Firefox window will open and Firefox will begin checking for updates. If updates are available, they will begin downloading automatically. If Firefox is already up to date, just close the About Firefox window.