Mozilla has released Thunderbird 2 Pre-Beta 1 onto their FTP. I have been using the latest nightly build since before Alpha 1 was released but I have only noticed one significant change and that is with the Options screen (pictured below). I am sure a lot has been done but it is probably more “under-the-hood.”
Here are the download links for Thunderbird 2 Pre-Beta 1 which are currently only available in English:
I hunted around to look for a release date on Thunderbird 2 Beta 1 but the most specific thing that I could find was “mid-September.” Since the Pre-Beta 1 was just released today I would treat that as a Release Candidate, which will normally float around for a few days before being marked as a release.
Here is what the Options screen now looks like: