The new free Mozilla Plugin Check service runs through your installed plugins to make sure they’re all up to date. This is for things like Java, Quicktime etc, not your addons. What they really need is an Add-on checker as well, but this will do for a start.
Why should I update my Plugins? * Old Plugins can interrupt browsing and waste your time. * Old Plugins increase your risk for attack by malware, viruses, and other security threats. * Updated Plugins have improvements that make the web better and safer for you.