Mozilla developers have released a new plug-in check service, which looks at more popular plug-ins and either warns users to upgrade whenever a newer version is available, or displays a green "Learn More" button if things are up to date. Furthermore, if there is a known exploit fixed by a newer version of a particular plug-in, you'll get a red button urging you to "Update NOW" -- or offering to disable it if there is no fix.
Mozilla's plug-in check service is in the testing phase right now but might eventually make its way onto the "What's New" page on future versions of Firefox. Users who want to check it out right now can do so by visiting this link, though. Note that your browser will detect an invalid certificate so you'll have to add an exception.