Should you update to Thunderbird ESR 140.12?
No: install Thunderbird ESR 140.17 instead. Thunderbird ESR 140.12 is a security update Mozilla released on 16 Jun 2026 for Mac, Windows and Linux. It fixes 29 security flaws, 0 rated critical and 12 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Thunderbird ESR 140.12.1, released 30 Jun 2026, which includes all of its fixes. If you’re on an older version, install that instead.
Should you update?
No: install Thunderbird ESR 140.17 instead. Thunderbird ESR 140.17 came out on 30 Sep 2026 and includes every fix in Thunderbird ESR 140.12, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Thunderbird ESR 140.12 still safe?
Not any more. The 6 updates since, up to Thunderbird ESR 140.17, fix 154 security flaws that Thunderbird ESR 140.12 doesn’t, and each became public the day its fix shipped. Installing it is safe: get it only from Thunderbird’s own About window, or thunderbird.net, never from a link in an email, text or pop-up.
What it fixes
Mozilla lists 29 security flaws, 0 rated critical and 12 high by Mozilla.[1] The 12 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-12289 | High | Privilege escalation in the Graphics: WebRender component |
CVE-2026-12290 | High | Memory safety bug fixed in Thunderbird ESR 140.12 |
CVE-2026-12291 | High | Use-after-free in the Networking: HTTP component |
CVE-2026-12292 | High | Incorrect boundary conditions in the Web Audio component |
CVE-2026-12294 | High | Sandbox escape in the DOM: Workers component |
CVE-2026-12295 | High | Sandbox escape in the DOM: Navigation component |
CVE-2026-12298 | High | Memory safety bug fixed in Thunderbird ESR 140.12 |
CVE-2026-12296 | High | Sandbox escape in the Security: Process Sandboxing component |
CVE-2026-12297 | High | Sandbox escape due to incorrect boundary conditions in the Networking component |
CVE-2026-12299 | High | JIT miscompilation in the DOM: Core & HTML component |
CVE-2026-12329 | High | Memory safety bug fixed in Thunderbird ESR 140.12 |
CVE-2026-12328 | High | Memory safety bugs fixed in Firefox ESR 115.37, Firefox ESR 140.12, Thunderbird ESR 140.12, Firefox 152 and Thunderbird 152 |
How to update, and how long it takes
A minute or two. Thunderbird restarts and your mail stays where it was.
- Open Thunderbird.On a Mac choose Thunderbird → About Thunderbird; on Windows open the ☰ menu → Help → About Thunderbird.
- Let it download the update.The About window checks for one by itself; when it says “Restart to update Thunderbird”, click it.
To check it worked: The same About Thunderbird window shows the version; an ESR version ends in “esr”. It should say 140.12.1 or later.
Before and after
- After: Thunderbird ESR 140.12.1, 30 Jun 2026, 2 security flaws
- Thunderbird ESR updates, every release and the latest version
- Updates, the latest version of everything HARD tracks
References
- ^ Mozilla. “Mozilla Foundation Security Advisory MFSA 2026-61”, 16 Jun 2026.