I have been using this add-on for several months now in conjunction with StrataBuddy, and I have to say that this is one of the cleanest variants on the Firefox UI that I have used.
The Firefox 4 Beta is now released, and the developer has stopped working on this add-on. However, so far this add-on is superior to the beta UI in terms of look-and-feel, usability, and overall polish.
Obviously, this add-on does not provide all the features than come with Firefox 4, but the implementation of tabs-on-top works more effectively with StrataBuddy and I feel that the Strata40 theme is closer in spirit to the Firefox 4 mockups than the beta.
Time will tell and the bugs in Firefox 4 will be fixed, but I don't believe this add-on has been rendered irrelevant.
Thanks to the team who created this add-on for realizing the goals of the 4.0 UI way ahead of its time!