Mozilla has launched the new site for Firefox Add-ons. This has a new and fresh revamped interface. There are new additions and features in this site. The add-ons which are in Sand box are also displayed along with the search result and labelled as experimental. Another feature is that it checks the version of Firefox you are running and disables add-ons installation if its not supported.
In case the add-on is not supported, this is the message you can see.
Since Firefox 3 is still in beta phase, many add-ons are not compatible with this version. Overall the new site looks well developed and arranged.