In today with their weekly Firefox update (er, ok - almost weekly), the 3.5.5 version fixes some "security issues". If for no other reason than to stop the annoying "Update Available" prompts, just do it and be done until next week. ;)
You know the drill - open Firefox, and if it doesn't automatically start downloading the update, click Help - Check for Updates. Follow the prompts - don't install anything additional like toolbars or security scans - and carry on with your day.
Or download fresh here: http://www.mozilla.com/en-US/