Starting with the Firefox 3.0.14 and 3.5.3 builds due out mid next week, Firefox will automatically warn you if you have an out-dated plugin. The first plugin to be checked will be Adobe Flash. Users will see a warning screen (see below), “You should update Adobe Flash right now” with a direct link to Adobe’s site to get the update when restarting the browser after installing the newest Firefox update. Users who have an up-to-date version of the plug-in will see the usual “You’ve been updated to the latest version of Firefox” screen (as seen at the bottom of this post).
Out-dated plug-ins are not just a security risk, but they can also cause performance issues (slowness, crashes, etc) with an updated version of Firefox. Trouble is, many people don’t update or don’t know how to update their plug-ins. Mozilla is going to be working with other plug-in vendors in the future so look for other update notices coming in newer releases.
Source: Mozilla Developers News