Betterbird. Simply better.

Betterbird is a fine-tuned version of Mozilla Thunderbird, Thunderbird on steroids, if you will.

Betterbird is better than Thunderbird in three ways: It contains new features exclusive to Betterbird, it contains bug fixes exclusive to Betterbird and it contains fixes that Thunderbird may ship at a later stage. Please refer to this feature table for examples. This should give you an impression of where the project is headed. More information on why we're doing the project can be found at the FAQ.

Betterbird 91.0 released on 5th August 2021

Today we are shipping the first release "bb8" of Betterbird version 91. In a successful beta period of Betterbird, we shipped the first beta release "bb1" on 12th July 2021, the last beta release "bb7" on 2nd August 2021. Please refer to the release notes for details.

Betterbird 91.5.0 released on 8th January 2022

Today we are shipping Betterbird version 91.5.0-bb24. Please refer to the release notes for details. German, French, Spanish and Japanese localisations available, other languages on request. As of 15th January 2022 we're also shipping a portable version for Windows.

Betterbird elsewhere on the web

Our GitHub repository github.com/Betterbird/
Discussion about all things Betterbird (in German) Forum @ www.thunderbird-mail.de
Interview with the project leader (in German) linuxnews.de
Arch Linux package aur.archlinux.org/packages/betterbird-bin/
Reddit
Handle with care, some posters there can't read or click a link
Betterbird - A fine-tuned version of Mozilla Thunderbird

Other news

For the much requested multi-line view, see here for details.

Examples of Betterbird being better in version 91.5.0-bb24

FeatureBetterbirdThunderbird
Features exclusive to Betterbird
Multi-line view like in Outlook, Lotus Notes and Postbox Yes!No, but requested since 2003
Can customise the header pane buttons Yes!No, feature deliberately removed
Quick Filter untagged messages Yes!No, development stalled since 2016
Permanently decrypt messagesYes!Not working since version 78, now only working in a beta version
Global search in encrypted messagesYes!Not working ☹️
Quick Filter search in encrypted messagesYes!Not working
Windows systray icon New messages count and folder listNew messages count
Printed attachment listShowing correct detailsNo details ☹️
Activity ManagerOpens in tabOpens in stand-alone window
Bug fixes so far exclusive to Betterbird(*)
Opening an attachmentFineSometimes missing from the list of Saved Files
Inserting links with spacesFineRandom incorrect results
View > Repair Text EncodingFixing subject and remembering repaired stateNot fixing the subject, need to re-apply
Various other bugs (see release notes)Fixed!Waiting, hoping, ... 🥱

Of course the Thunderbird team is busily adding a whole bunch of much welcomed new stuff and improvements, but making sure that everything remains in working order falls by the wayside. Thunderbird is shipped with unnecessary regressions, so things that used to work stopped working.
Best example: 91.3.2 shipped with broken Calendar printing 😩
(*) We supply our changes to Thunderbird, but it is unclear if/when "upstream" will accept them.