Didn't Firefox have that same check page for the plugins built into the addons manager in the browser before? Guess it was in FF3 and they took it out in 4.
Anywho, you could also just select the plugins in addon manager, and check for Updates, or apply updates automatically.
I find it more accurate to just check the specific software site that has a plugin I use. Being that it can take a long time for Mozilla to update their page of plugins (and even the addons) with a new version, when the main site of the software has had a newer version out for some time.