Software Update: Mozilla Thunderbird 91.0

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The Mozilla Foundation has released version 91 of Thunderbird. Thunderbird is an open source email and newsgroup client, with features such as support for several mail and news accounts, a spam filter, spell checker, and a customizable appearance. In version 91 has included a warning when an email is sent to a possibly non-existent address, or when there are too many addresses on the public address line. There is also support for CardDAV and there is a native client for a Mac equipped with an Apple Silicon processor.

At the moment it is not yet possible to do an automatic update from version 78 to 91. An existing installation can be updated by manually downloading and running the installation file. The automatic migration will be added in a future version. The complete changelog for this release can be found below.

What’s New

  • Native support for macOS devices built with Apple Silicon CPUs
  • Thunderbird now operates in multi-process (e10s) mode by default
  • Latvian language support
  • New user interface for adding attachments
  • Enable redirect of messages
  • Ability to change order of accounts in UI
  • Allow showing empty CC/BCC rows in compose window
  • Warning popup when sending a reply to a likely non-existant email address such as “[email protected]
  • Warning popup when public recipients of a message exceeds threshold
  • Add support for “X-Unsent: 1” header, to open a saved email in a compose window for editing
  • Add support for non-ASCII characters in recipient addresses
  • Context menu to expand mail list pills in the compose window to the list of recipients
  • Quick Find is now available in the multi-message (thread summary) view
  • Keyboard shortcuts to access To/CC/BCC fields of compose window
  • Allow pinning folder views to the Folder Pane
  • Language packs and dictionaries added to about:support
  • PDF.js viewer now included in Thunderbird
  • OpenPGP: “Copy Key Id” option added to Key Manager context menu
  • OpenPGP: Added config option to disable encryption saved drafts
  • Encrypt mail to BCC recipients (with warning that it will expose the recipients in the list of keys)
  • CardDAV address book support
  • CardDAV address books automatically detected based on provided user information
  • Access to Outlook Contacts; To enable, set ldap_2.servers.outlook.dirType to 3; This setting may cause startup delays.
  • Suggest replacements for discontinued/incompatible add-ons
  • Chat: Beta-level support for Matrix servers (set chat.prpls.prpl-matrix.disable to false)
  • Calendar: Remote calendar auto-detection now supported
  • Calendar: Calendar and category colors now displayed in selection dropdowns
  • Calendar: “Edit” item added to event context menu
  • Calendar: Support opening of .ics files by double-click
  • Calendar: Thunderbird now informs the operating system that it knows how to open webcal: URLs
  • Calendar: Filter and sort items to be imported in the Import dialog
  • Prompt to choose the identity of an accepted calendar invite when no identity matches the event attendees list
  • Support mid: URL scheme for Related Links in calendar event dialogs
  • Permissions to access calendar and address book are now requested when a GMail account is set up, so that calendar and address book can be accessed without having to re-authorize (see notes)
  • Calendar: Per-calendar and global notification settings (in addition to alerts set in events)
  • Tasks: Undo/Redo support for event and task creation and deletion


  • “Master Password” renamed to “Primary Password”
  • Renamed “Add-ons” to “Add-ons and Themes” and “Options” to “Preferences”
  • Account setup moved to a tab
  • Mail recipients that are not found in any address book will no longer appear in red type; invalid addresses will appear red
  • Clicking on an already-selected pill in the recipient list will now allow editing the address
  • Folder pane color scheme overhauled with a focus on readability
  • Sending backend, SMTP protocol, and LDAP protocol implementations rewritten in Javascript
  • Sending backend, SMTP, and IMAP protocols now operate only in UTF-8 mode
  • Sending a message will now fail if any recipients are not accepted by the SMTP server
  • Error messages from an SMTP server are now displayed to the user
  • UI Customization controls moved to the View menu
  • End-to-End Encryption “Advanced Settings” are now disabled when encryption is not configured
  • Movemail support removed
  • WeTransfer FileLink provider removed
  • Enterprise policies updated
  • Printing UI updated
  • AutoComplete from LDAP directories now searches by substring instead of left-side match
  • Chat modules and custom widgets are now lazy-loaded to improve Thunderbird startup time
  • Default IRC server for new chat accounts changed to “”
  • Chat: Image based emoticons replaced with Unicode
  • Calendar: Opening an existing event now opens summary dialog
  • Calendar: Default to CalDAV when supported by the server


  • Thunderbird did not properly handle Self-signed certificates on IMAP servers
  • Various issues with special and non-ASCII characters in IMAP folders names, especially on Gmail
  • Drag & drop operations could be disrupted by incoming mail notifications
  • Dragging a folder from an authenticated IMAP server to a not-yet authenticated server fails
  • Temporary errors from POP3 servers would cause Thunderbird to stop downloading mail until it was restarted
  • Favorite Folder view did not maintain UI state between Thunderbird restarts
  • Saved search virtual folders were not retained after restarting Thunderbird
  • Thunderbird did not correctly warn about all affected filters when removing a folder
  • Manually running filters on a maildir folder did not work
  • Partially downloaded messages displayed in a stand-alone window did not refresh after clicking link to download the rest of the message
  • An erroneous “Sender” header was displayed in the message preview after viewing certain S/MIME signed emails
  • Some temporary files created by the message composer were not automatically removed after closing the compose window
  • Archiving messages from an NNTP account made the target folder unusable
  • Compose window: The user-configured style for quoted text was not honored
  • Message headers in compose window disregarded manual resizing when adding additional recipients
  • Various improvements to new message notifications
  • A Thunderbird icon pinned to the Windows taskbar reverted to a non-functional placeholder after an update
  • Various improvements to recipient pills in message compose window
  • Keyboard shortcut to open the Message Security popup did not work on Mac
  • Subject column in message lists sometimes showed text from invalid email headers
  • Account settings: When creating a new account, some already entered settings did not copy to the Advanced Config dialog
  • Default address book preference was not honored in the contacts sidebar
  • Import Address Book from CSV did not allow mapping all available fields
  • LDAP address books did not display multi-valued attributes
  • Address Book: Users with LDAP admin rights could delete accounts if trying to delete an entry from an address book backed by LDAP
  • Some preferences related to an address book were not removed when the address book was deleted
  • Shortcut for Advanced Address Book Search (Ctrl+Shift+F) did not work on Linux
  • Radio buttons could not be selected on photo tab of an address card
  • Windows uninstaller did not always remove all Thunderbird program files
  • Chat: Double clicking an account type in new account wizard did not select it
  • Chat account settings did not always save as expected
  • User nickname colors were not used in chat content
  • Chat: Multiple system messages did not collapse
  • Calendar: Reminder details appeared editable when viewing an event
  • Calendar: HTML rendering in event descriptions restored
  • Calendar: The toolbar in the invitation details dialog was not honoring the theme colors
  • Calendar: Various dialog updates
  • Calendar: Import and export via CSV did not parse years correctly in event dates
  • Calendar: Event boxes were not always focused when clicked
  • Dragging and dropping an ICS file to the Today Pane did not populate the “New Event” dialog
  • Calendar: Reduced flickering effect in Today Pane
  • Calendar: The time of day indicator line did not update automatically
  • Location field was not preserved when modifying recurring events stored on a remote calendar
  • Today was difficult to pick out visually in the month/multiweek calendar views
  • Columns in Today Pane were not resizable
  • Calendar event text could render outside the confines of the event block
  • Event time and event name were not vertically aligned
  • Improved formatting of event descriptions with long links, such as Zoom invites
  • RSVP replies to invitations sometimes sent from the wrong email address
  • Various UI and theme improvements, especially to dark themes
  • Various security fixes

Version number 91.0
Release status Final
Operating systems Windows 7, Linux, macOS, Windows 8, Windows 10
Website Mozilla Foundation
File size


License type Conditions (GNU/BSD/etc.)
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