Mozilla has officially released Thunderbird 27 Beta 1, an email and RSS client, for all the available platforms, including Linux.
Thunderbird developers have decided to release a new Beta version, skipping from the 26.x branch to the 27.x branch without having a stable release in between.
The good news is that you can download and run the new build without removing the old one and without having to install it.
You can check the list of changes when it becomes available but, for now, you only get a 404 error. Most likely it's all about security updates. Don't forget to check out our review of the RSS function of Thunderbird.