Rated 5 out of 5 stars
It's probably the best one out there. However, It needs updating as Google has recently changed it's url for obtaining page ranks. Most third party toolbars (This one included) have yet to catch up.
We are aware of the changes and already fixed the issue. Mozilla team is reviewing the new version of addon, it will be available ASAP. But still you can install addon Ver 4.1(which is under review) from the url https://addons.mozilla.org/en-US/firefox/addon/webrank-toolbar-pagerank-alexa/versions/