Thunderbird 78.9.0 fixes five security issues and resolves a few bugs

The Thunderbird email app received a new stable version 78.9.0. It comes with bug fixes, and also includes patches for five security issues with high and moderate severity.

Mozilla Thunderbird Banner

Thunderbird is my preferred email client of choice. I use this app on every PC and on every operating system I use. It is stable, contains all the features you need, supports add-ons and also comes with a useful RSS reader. I am using Thunderbird for many years and never felt the need to look for an alternative.


Thunderbird 78 doesn't support classic XUL add-ons any longer, but includes some of their features out-of-the-box. For example, on Windows you can minimize the app to the system tray.

What's new in Thunderbird 78.9.0

Security fixes

  • CVE-2021-23981 (HIGH): Texture upload into an unbound backing buffer resulted in an out-of-bound read
  • MOZ-2021-0002 (HIGH): Angle graphics library out of date
  • CVE-2021-23982 (MODERATE): Internal network hosts could have been probed by a malicious webpage
  • CVE-2021-23984 (MODERATE): Malicious extensions could have spoofed popup information
  • CVE-2021-23987 (HIGH): Memory safety bugs fixed in Thunderbird 78.9

Other fixes in this release resolve a number of bugs found in previous app versions, and applicable to the address book, add-on manager, calendar, and the user interface of the app.


  • New mail notification displayed old messages that were unread
  • Spaces following soft line breaks in messages using quoted-printable and format=flowed were incorrectly encoded; existing messages which were previously incorrectly encoded may now display with some words not separated by a space
  • Some fields were unreadable in the Dark theme in the General preferences panel
  • Sending a message containing an anchor tag with an invalid data URI failed
  • When switching tabs, input focus was not moved to the new tab
  • Address Book: Syncing a read-only Google address book via CardDAV failed
  • Address Book: Importing VCards with non-ascii characters would fail
  • Address Book: Some values may not have been parsed when syncing from Google address books.
  • Add-ons Manager did not show if an addon used experiment APIs
  • Calendar: Removing a recurring task was not possible

Finally, the release notes include a single known issue. The fix for the bug "new mail notifications not showing if unread messages from previous notifications were still there" has been reverted, as the devs are still working on resolving this issue.

Download Thunderbird

Thunderbird users should eventually get the update automatically via the built-in update mechanism. To update it manually, select menu > Help > About Thunderbird to open the "About" dialog and initiate the update check. After that, the app should download and install the most recent version available.

Also, you can manually download the app from its official web site:

Download Thunderbird

The release notes are available here.

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Author: Sergey Tkachenko

Sergey Tkachenko is a software developer who started Winaero back in 2011. On this blog, Sergey is writing about everything connected to Microsoft, Windows and popular software. Follow him on Telegram, Twitter, and YouTube.

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