If you're a Thunderbird user, you're probably familiar with what the open-source mail manager can already do, so here's an edited shortlist of notable new features and improvements:
New Search with Advanced Filtering Tools: Search results now include advanced filtering tools. You have the option to filter your results by sender, tag, attachments, people, folder, and mailing list. You can also filter your email using the timeline tool.
Smart Folders: The folder pane offers a Smart Folders mode which combines special mailboxes, like Inbox, from multiple accounts. Smart Folders is now on by default.
Improved Gmail Integration: Better recognition and integration of Gmail's special folders such as Sent and Trash including non-English versions of Gmail. Thunderbird also uses All Mail as the Archives folder.
Improved Gmail integration is definitely a welcome improvement