One very nice addition to Thunderbird 3.1 is it's filtering capabilities. I use a couple of accounts and previous versions were always hard to search. This has changed considerably. You have a Quick Filter at your disposal for for instance Tags, Starred etc. There still is the general that was already awesome and in individual mailboxes there is filtering for Sender, Subject etc. The most valuable addition however is that you can keep the filters applied when switching mailboxes or folders.