When Mozilla moved Firefox to a rapid release cycle, committing it to producing a new version number every three months or so, it decided to move Thunderbird – Firefox’s email companion – to the same cycle.
This means three – even four – different versions of Thunderbird are now in existence at the same time: the final, stable release that will appeal to most users, plus various flavours of pre-release versions, made available to experienced users and developers for testing.
The Beta version of Thunderbird is the version closest to release - just one month away from being launched as the Final build, but it's still not yet quite ready for prime time. You can expect some stability issues with this build, which is why it’s always a good idea to regularly back up your email (use a tool like MozBackup), but on the whole you should find the beta build is stable enough for day-to-day use.
With the move to rapid-release cycles, the number of major new features are understandably thin on the ground; instead expect to find a program that’s marginally quicker than the current stable build, boasting better support for standards. On the flipside, you may find some add-ons no longer work with the new version, although many plug-in developers take steps to ensure their add-ons work with the beta build ahead of the final release.
What can you expect in version 9.0? Based on the Gecko v9 rendering engine, added an opt-in system for sending back usage data to Mozilla (presumedly so Mozilla can tweak Thunderbird in future releases), better keyboard handling for attachments and the ability for Windows users to hide the menu bar. Other than this, various unspecified platform fixes and enhancements, as well as minor UI improvements.