This article describes the major changes that are visible to users in Thunderbird version 38. Full details of all the changes can be found in the Thunderbird 38 release notes.
Table of Contents
Chat supports Yahoo Messenger
Thunderbird chat now supports Yahoo Messenger.
Filtering sent/archived messages
You can now filter on sent and archived messages.
Search multiple address books
You can now search multiple address books!
Expanded Folder Pane columns
In the folder pane (the list of folders on the left), you can now show the column picker for expanded options (Unread, Total and Size) without using the Extra Folder Columns add-on, which is now incompatible. To show the column picker using the traditional menu bar, select . Or, click the toolbar menu button 
and then select .
Google OAuth2
Thunderbird 38 now supports the OAuth 2.0 protocol now used by Google for authentication.
Calendar integration
Starting in Thunderbird 38, the Lightning calendar add-on is bundled with Thunderbird.
Mail Composition Fixes
Some idiosyncrasies in the message compose window have been addressed: The composition font is no longer lost after clicking elsewhere in the message and then returning to the end of the line of the text being entered. Also, the composition font is no longer lost after pasting a bitmap from the clipboard. The inline spell checker no longer loses red underlines.
Inline spell checking: New messages will now always be spell-checked in the language selected in and not as before in the last language used. Also, the spell checking language is now always synchronised between subject and message body. You can even compose messages in different languages concurrently now without having to switch dictionaries.
Font Handling on Linux
Linux provides system fonts "sans-serif", "serif" and "monospace". These font names conflict with the generic CSS font family names. Therefore and for compatibility with Thunderbirds on other platforms, the ability to compose messages using these three fonts was removed on Linux. Users who had selected one of these fonts as their default composition font have to select a different font. Instead of "sans-serif", "serif" or "monospace" users should select "Helvetica, Arial", "Times" or "Courier" (or "Fixed Width") respectively.
The three aforementioned Linux system fonts can still be used as default display fonts.
