The Mozilla Thunderbird team has released version 159 of its open-source email client. For Linux users, the most noticeable addition is an improved system tray indicator showing whether unread messages are available. The tray icon’s tooltip also shows the unread message count, making it easier to check for new mail without opening Thunderbird.
Another addition concerns Microsoft Graph integration. Thunderbird now automatically detects Graph calendars when creating a new account, simplifying setup for users who rely on Microsoft’s email and calendar services.
Beyond these two features, Thunderbird 159 addresses several issues affecting Microsoft Graph and Exchange accounts. The update fixes excessive Graph API requests that could trigger throttling and delay message display. It also resolves account setup failures caused by incomplete outgoing server settings and restores the Edit Configuration functionality for Exchange and Graph accounts.
Email handling receives numerous fixes. POP3 NTLM authentication now works correctly, while IMAP namespace root folders are no longer inaccessible due to duplicated folder names. The release also corrects problems with non-ASCII IMAP tags that could exceed server length limits.
Attachment handling has been improved as well, including fixes for MP4 files truncating when forwarding messages and for Save All and Detach All operations failing on attachments with invalid filename paths. Other corrections prevent inline images from disappearing when editing local or POP messages as new.
Regarding calendar and contact synchronization, CalDAV calendars no longer stop syncing after network errors until Thunderbird is restarted, while OAuth2-based calendar discovery has been fixed. Additionally, CardDAV users benefit from corrections to repeated password prompts and synchronization failures caused by invalid sync tokens.
On the performance side, the update resolves a startup hang when restoring IMAP message and PDF tabs. It fixes excessive CPU consumption caused by malformed message database files and prevents large ICS calendar files from exhausting memory and freezing the application.
It also addresses several security-related issues, including Unicode email addresses that could conceal different Reply-To destinations, SMTP addresses with line breaks that could inject unintended commands, and RSS feeds that could inject arbitrary message headers.
For more information, see the release notes.
Thunderbird 159 is available for Linux, Windows, and macOS. Existing users can check for the update through the application’s built-in update mechanism, while Linux users who installed Thunderbird through their distribution’s package manager will receive it according to their distribution’s update schedule
