Mozilla just released Alpha3 of Thunderbird version 3; my advice: don’t bother.
It’s got a few mildly graphically improved widgets, but no other noticeable feature improvements (so far).
BUT … it completely breaks online/offline operation, and refuses to download ANY emails unless you manually download EACH AND EVERY email by clicking on it.
This makes it, well, entirely useless for anyone using it as an offline mail client (which, I’m assuming, is the main use case for most people!)
So, don’t bother downloading it – stick with Shredder 2 / Thunderbird 3 alpha 2. It’s buggy, but at least it does download email when you tell it to! (usually).
NB: in case anyone wonders why I’m using an alpha/pre-alpha in production, here’s why: On OS X (and, yes, this is partly a reflection that OS X is a second-citizen when it comes to TB releases), Thunderbird 2 is so utterly useless that the 3.0 pre-alpha works better *across the board*. I was rather depressed to discover that TB 2, gold version, is missing many of the core features of Mozilla Mail, which TB 1 was based upon. TB 3 at least manages to bring back some of the basic functionality (although not all, not yet – I have fingers crossed!). In particular: where TB 2 cannot cope with Gmail (apparently because Gmail willfully ignores bits of the Internet Standards; sigh – BAD google!), for TB 3 the Mozilla devs hacked around Gmail’s bugs to make TB 3 work with Gmail *mostly* correctly.
NB2: Mozilla Mail was the best graphical mail client I’ve ever used, even today some years after it got replaced by Thunderbird. There is (was) some great usability stuff in there. So, much as it is frustrating to be stuck with an alpha build right now, it’s still pretty good, and I’ve got high hopes for the final release of TB 3!