Windows only: Mozilla released the mockups for the far-in-the-future Firefox 4.0 release, and just like with the 3.7 release mockups, a motivated user has already created a lookalike theme that you can install now.
(Click the image above for a closer look.)
Installing this theme isn’t quite as simple as you are probably used to—you’ll need to first enable the All-Glass Firefox extension for the Aero effects, the Personal Menu extension to hide the menubar and add the Tools button, the Stylish extension for a tweak that fixes the text, and then drag the mockup theme’s *.jar file into the add-ons window to install it. Next, you’ll need to install the Fission extension that combines the address bar and progress bar, and the Omnibar extension to combine the address and search bars together.
Once you’ve successfully completed all of the steps—which are detailed on the download page—you should have an impressive browser style that looks very similar to the screenshot.
Strata40 Beta v0.2 [deviantART]