I've been loving Postbox. It's based on TB, but with a better UI and great Gmail/Google Apps integration. It has a free 30-day trial, and will import your accounts from TB seamlessly.
if Mozilla wanted to do "good things" and not "lets try to be another corporate company!", they' buy off postbox and release that open source, and keep the devs working on it.
They're abandoning TB because it sucks. Frankly. It's buggy, leaky, etc. No one really cared for it, for too long.