Today we're starting our beta program for Betterbird 153 with 153.0b1-bb1. Please refer to the Release Notes for full details.
We're shipping this only in English and for Windows. Language packs for other languages are available.
Note that we're just starting to gain experience with the new 153 series, so please do not use this version in production yet. Also note that Thunderbird migrated password storage from logins.json to logins.db, so when downgrading, you need to re-enter all your passwords (or restore the previous logins.json file).