I've reported that after a period of initial stability, the Beta has entered a period where it has been crashing several times a day. Even when not crashing, it can't walk and chew gum at the same time much of the time, or get email and have you write one at the same time.
Further, the longer it's open, the worse the problem gets and it starts to overheat when it gets hits over 100,000 K of memory use.
As the picture shows (Every picture tells a story don't it), Thunderbird was overheating this morning when I took this picture, and I was not surprised when it crashed soon after. This is not a new issue and I'm hoping (I'm dreaming, I'm praying) that the programmers fix this soon.
4/28/08 Note: I noticed a ton of traffic to this post today and I just want to point out that I posted this more than a year ago. I've since stopped using Thunderbird, so I can't say if it's still an issue. Just wanted new readers to be aware of this.