If like me you're wondering when this will be released via the in-application auto-update mechanism, the bottom of the article states:

> As with any major release, we sometimes become aware of corner cases after significant public testing. The Thunderbird team will wait to enable automatic updates until we’re confident no such issues exist.

So if you want it now you have to download the new installer, you can't just do what I was doing and continually check for a new version and hope it turns up in the next few hours :)