What's New in Mozilla 1.7a? Well, better pop-up blocking, multiple identities in mail, UI for smooth scrolling, 2% decrease in binary size, 4% improved pageload times, better dynamic style changes, and new SVG backend for a start.
The multiple identies thing would be nice in Thunderbird... currently the only way is to add a dummy POP/IMAP account, so that when sending mail you can pick who you want to send as. (I have multiple virtual domains that I send as, but it's all the same inbox)
SVG anything is always cool of course.