Should you update to Thunderbird ESR 140.13?
No: install Thunderbird ESR 140.17 instead. Thunderbird ESR 140.13 is a security update Mozilla released on 21 Jul 2026 for Mac, Windows and Linux. It fixes 33 security flaws, 2 rated critical and 16 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Thunderbird ESR 140.14, released 18 Aug 2026, which includes all of its fixes.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 140.13, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Thunderbird ESR 140.13 still safe?
Not any more. The 4 updates since, up to Thunderbird ESR 140.17, fix 119 security flaws that Thunderbird ESR 140.13 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 33 security flaws, 2 rated critical and 16 high by Mozilla.[1] The 18 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-15718 | Critical | Invalid pointer in the JavaScript: WebAssembly component |
CVE-2026-15719 | Critical | Site isolation issue in the DOM: Navigation component |
CVE-2026-16349 | High | Same-origin policy bypass in the DOM: Navigation component |
CVE-2026-16350 | High | Incorrect boundary conditions in the Audio/Video: cubeb component |
CVE-2026-16362 | High | Use-after-free in the WebRTC: Audio/Video component |
CVE-2026-16351 | High | Sandbox escape due to use-after-free in the DOM: Navigation component |
CVE-2026-16352 | High | Sandbox escape due to use-after-free in the Disability Access APIs component |
CVE-2026-16363 | High | JIT miscompilation in the JavaScript: WebAssembly component |
CVE-2026-16353 | High | Invalid pointer in the DOM: Bindings (WebIDL) component |
CVE-2026-16354 | High | Information disclosure in the Graphics: ImageLib component |
CVE-2026-16368 | High | Incorrect boundary conditions in the JavaScript: WebAssembly component |
CVE-2026-16369 | High | Integer overflow in the JavaScript: WebAssembly component |
CVE-2026-16355 | High | JIT miscompilation in the JavaScript Engine: JIT component |
CVE-2026-16356 | High | Sandbox escape due to use-after-free in the Disability Access APIs component |
CVE-2026-16357 | High | Incorrect boundary conditions in the Graphics component |
CVE-2026-16412 | High | Memory safety bugs fixed in Thunderbird ESR 140.13 and Thunderbird 153 |
CVE-2026-16360 | High | Memory safety bugs fixed in Thunderbird ESR 140.13 and Thunderbird 153 |
CVE-2026-16361 | High | Memory safety bugs fixed in Thunderbird ESR 140.13 |
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 140.14 or later.
140.12.1 vs 140.13
Thunderbird ESR 140.12.1, released 30 Jun 2026, fixed 2 security flaws. Thunderbird ESR 140.13 fixes 33 security flaws and includes the fixes before it, so going straight to 140.13 gets you both.
- Before: Thunderbird ESR 140.12.1, 30 Jun 2026, 2 security flaws
- After: Thunderbird ESR 140.14, 18 Aug 2026, 31 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 2026-72”, 21 Jul 2026.