@OXMichaelJust curious, are you using the Vertical View? If so I have (mostly) good news: it's an existing bug that's already been fixed in the newest Daily build. So you should see it working normally in the very near future. Thanks for commenting!
I have been using Tbird for about a year and throughout all of that time sometimes drag and drop of emails does not work. They just don't move, very annoying. The failure will happen many times and then will start working again and probably work for a few items and then stop again.