Windows/Mac/Linux: The first pre-beta build of Firefox 4 hit servers today, bringing several changes to the veteran browser, including a new tabs-on-top interface, a refreshed overall look and consolidated menus.
The update's still rough around the edges, and as a pre-beta, it's a very early-adopter download only. But if you're eager to play with the bleeding edge of Firefox, you can download the release (Mozilla appropriately names their nightlies "Minefield") here. (Look for the appropriate download for your platform with a name like firefox-4.0b2pre.en-US.)
Firefox 4.0 pre-beta nightly builds [Mozilla]