Windows/Mac/Linux: It doesn't look that different, and the version number is certain to change, but Mozilla is offering a "Developer Preview" of the rendering engine inside Firefox 3.7, which will actually become 4.0. If you're down with unstable alphas, it's all yours.
The release is technically a first alpha of the web rendering engine inside the next Firefox release, Gecko 1.9.3, but the browser they're releasing it with compiles as Firefox 3.7 Alpha 1. Not tab or interface changes here, but the browser itself incorporates things like native WebGL 3D graphics, advanced CSS capabilities, and other code and compatibility changes detailed at the link below.
It's rough, it's unofficial and it shouldn't be your primary browser, but if you want the earliest possible look at what's coming in Firefox, go ahead and grab it — or wait for the inevitable portable version to follow.