Thunderbird kept crashing on me on Leopard, at least three out of four times that I launched it. At first I thought it was Enigmail, because it seemed that the crash was if I attempted to view an encrypted message while it was still loading mail from the servers, but testing that hypothesis revealed it to be false. Much mucking around OS X got me to the ~/Library/Cache/Thunderbird/Profiles, which contains some files supposed to help with a fast launch. I deleted the whole thing (Cache directories are expected to be regenerated by the application), and that has Thunderbird working back to normal.
This might be related to the other Leopard problem I’d mentioned. If you’re having Thunderbird problems, I suggest you start by killing the cache.