I did a review of the latest Thunderbird beta back in December and I was SO impressed by its stability. I also wondered in the review if they could move through the Beta cycle without losing that stability. Unfortunately, Mozilla hasn't been able to do that.
I'm finding that Thunderbird crashes several times a day, sometimes a couple of times in the same hour. When it isn't going through a full crash, it sometimes just locks up for a time, leaving me to wait to read an email. It also has an annoying habit lately that when I'm writing an email, it loses the focus of the email window and I have to alt-tab to find it.
I did a little check under the hood and noticed that Thunderbird is using more than 100,000 K of memory after it has been running for a time. It seems to have an eerily similar memory leak problem as its cousin, the Firefox browser. When I first start Thunderbird it's using about 60,000 K, but over time it jacks up to over 100,000 and this seems to be when it crashes.
I know you take your chances when you use Beta software, but honestly, I'm getting annoyed enough to consider giving it up completely. I'm just hoping that Mozilla gets its act together and fixes these issues before it's released and saves me the trouble of selecting a new email program.