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.
This is the latest Thunderbird beta, now v66 includes these improvements (see full changelog):
ADD-ON SUPPORT: Restartless add-ons continue to work if their authors have made the necessary minor adjustments. Non-restartless legacy add-ons using XUL overlays are only supported if add-on authors have adapted them. Apart from Lightning/Calendar there are a few that have been adapted, like "ThunderHTMLedit", "Compact Header" and "Signature Switch". Instaling legacy add-ons now requires a restart. Legacy add-ons can now specify an options page again.
DICTIONARY SUPPORT: Only WebExtension dictionaries are supported now. Please download new dictionaries from addons.mozilla.org since addons.thunderbird.net still provides "legacy" dictionaries compatible with Thunderbird 60 ESR. Note that Thunderbird 60.5 ESR can also handle WebExtension dictionaries.
Maildir now uses the message ID as file name and "eml" as file extension
Tags set on IMAP folders sometimes not visible to other users
Various theme fixes and improvements