Mozillazine has links to various places to download it. You might be asking how could 1.3a come out so soon after 1.2. Well, 1.2 branched off the main trunk a couple of months ago (August? I forget) and lots of stuff has been getting added to the main trunk since it did. So when 1.3 comes out as an alpha it really has a lot more feature changes to it than you'd expect given the time interval between 1.2 and 1.3a. I haven't downloaded it yet. The release notes don't have anything particularly exciting in them (at least to me - though if you use Moz mail and news you might feel otherwise) and so I'll probably wait till 1.3b before trying out 1.3 builds.