The latest Firefox updates, v3.5.3 and v3.0.14, will now check to make sure you are running the most recent, most secure version of Adobe Flash Player. If you should get this notification after upgrading, be assured that it is a legitimate popup. You should then update your Flash Player as soon as possible as there are many known vulnerabilities in older versions that can be exploited to install malware. This entry in the Mozilla Security Blog explains the new feature:
http://blog.mozilla.com/security/2009/09/04/helping-users-keep-plugins-updated/