Thunderbirds Argot
The Mozilla team announced Wednesday that the Thunderbird RC1 is now available. You can read the announcement here and follow the link in the post to the download. From what I can see in the Mozilla support forums, the Thunderbird e-mail client is set to go gold sometime next week (barring unforeseen problems with the RC1).
With such a short time before then, I would say it's best to wait until next week before downloading unless, of course, you want to help the Moz Squad test the RC1.
Speaking of Moz, columnist Brian Livingston's latest newsletter has a couple of tips on tweaking Firefox to run faster. He also has a link to a guy who has created optimized builds of Firefox based on your CPU. YMMV. Professional driver on a closed course. Don't do this at home (go to a friend's house and try it there).