Well, Firefox automatically does that for you. By default, it checks for updates to your themes and extensions. And once Firefox 3 reaches to the release state, there are no more changes to the API until Firefox 4, and so your extensions should work with every minor update. Of course, there are lazy coders out there that check for specific builds of Firefox, because they seem to ignore what a wildcard is
Also, Firefox 3 allows you to install extensions from the addons window, but I ignore if such installation actually checks for version compatibility before installing, that is, filtering the results by the returned version.
And yes, in the case you want to track updates, WebSite-Watcher or a similar app with monitoring capabilities like Newzie would help.