Betterbird is an enhanced version of Thunderbird with new features and bug fixes not available in Mozilla Thunderbird.
Betterbird looks and feels like Thunderbird, but it's easy to spot the new features available. Many of these features have been requested for over a decade, and some have been attempted and failed. Examples include:
Multi-line view like in Outlook, Lotus Notes, and Postbox Customise the header pane buttons Complex search terms Quick Filter untagged messages Permanently decrypt messages Global search in encrypted messages Quick Filter search in encrypted messages Notification of new messages in newsgroups Configurable display of addresses in the message list And more