The alpha build is the first of several developer (and enthusiast) previews, and is code-named Namoroka, after a national park in Madagascar.  Firefox 3.6 will likely see a final release late this year.

The 3.6 build introduces the new Gecko 1.9.2 rendering engine.  The new engine packs improved JavaScript performance as well as improving overall rendering of text and graphics, for faster page loads.  Multiple background images are now supported, and CSS3 has also been tweaked.  Perhaps most interestingly, the Tab Preview feature (similar to Aero Peek in Windows 7), which had been removed in the final beta of Firefox 3.5, has reappeared.

For developers, the chromedir attribute has been replaced with a pseudoclass, a new focus model has been introduced, and Gecko now has been moved to using one native widget per top-level content document.

Like any alpha build, there's likely numerous bugs and stability issues -- so consider yourself warned.  But if you want the browser anyway, head on over to the Mozilla Developer Blog to grab it in Linux, OS X, or Windows forms.  The site can also be used submit your experiences and bugs that you've found during your testing.