In my years of using Thunderbird, there have been a few notable transition points in my personal experience with it. One that I remember well is when we optimized deletion to be asynchronous, which completely got rid of the delay in deleting messages. Another that I’m happy to be able to share now is Jonathan Protzenko’s Conversations add-on, now available through this Labs blog post. It’s remarkable how well Jonathan was able to take an early mockup from a couple of years ago and carry it through to a very, very useful and pleasant and quite full-featured conversation view. Try it out, if you’re like me you won’t want to go back.
On a technical note, it’s nice to see how the various bits of the refactorings and new technology pieces we’ve been building into Thunderbird can be used to build compelling new UIs.