UPDATE: Firefox 4 Beta is out. Check it out here.
So we've seen mockup themes for Firefox 4.0, looking cleaner and less intrusive with the Aero interface for a sense of elegance. Firefox 4.0 seems to be quite a long way ahead (scheduled for November 2010), but we can get approximately the same look now with Strata40, for Firefox 3.6+.
The theme must be at least coupled with StrataBuddy, which gives you some extra features planned for Firefox 4.0, such as the App Button and the Aero interface (XP users can use the Default/Luna/Royale/Silver interface). Either way, the theme looks very much at home with Firefox, that people can mistake it as the new official theme.
For a fuller experience, install Fission, Tab Progress Bar, and App Tabs as well. Fission gives you the streamlined progress bar under the URL bar, and Tab Progress Bar gives you the progress bar on top of the tabs, both with the help of StrataBuddy. Should it not look like the one in the screenshot above, go to Theme Options (its inside the orange App Button), look under Misc, and check the Progress lines. You may need to disable and re-enable Tab Progress bar in order to get the right look. App Tabs allows you to pin favicon sized tabs, which saves tab space and quite handy for frequently opened websites. It is also a proposed function in Firefox 4.0.
I put on a few more configurations to make it look better (and less intrusive). I've removed the Menu Bar (the App Button replaces it), Bookmarks Toolbar (I don't use it), and the Home button (replaced by the Home tab). Other than that, most configurations are available inside the Theme Options. Go and experiment.
So far I've been using this theme, it doesn't slow down Firefox. Even if they do, its not really noticeable. My verdict? Recommended.
Also: Strata40 Lite to retain compatibility with some other extensions, as Strata40 is known to break compatibility with some extensions. Should be coupled with AeroBuddy for the Aero interface.