Just as Firefox has been moved onto an accelerated release schedule, so Mozilla has increase the speed at which updates are released for its email client, Thunderbird. At any one time there are three different versions of Thunderbird available – the stable release, the beta version of the following release (this version of the application), and the alpha preview of the subsequent version called Earlybird.
For day to day use, it is recommended that most people stick with the stable release that is made available in the standard release channels, while those who like to live right on the cutting edge and experience all the latest feature may like to take a look at the Earlybird alpha. The problem with alpha software is that it can often be unstable, but by opting to work with the beta version of Thunderbird, you get the best of both worlds; a sneak peak at what is to come in future versions, but nothing so experimental that it is unreliable.
It is important to note that while the Earlybird alpha can be installed alongside an existing copy of Thunderbird, installing the beta will overwrite any stable version that is already installed. This beat version is primarily a bug fixing release and many of the problems that have been found in version 13 of the email client have now been fixed.
After Mozilla's recent announcement that Thunderbird is entering its "extended support phase", which basically means it'll only be officially maintained with minor bug fixes and security updates going forward.
The Beta version of Thunderbird only brings small incremental fixes and improvements.