Should you update to Thunderbird ESR 140.15?
No: install Thunderbird ESR 140.17 instead. Thunderbird ESR 140.15 is a security update Mozilla released on 1 Sep 2026 for Mac, Windows and Linux. It fixes 14 security flaws, 0 rated critical and 10 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Thunderbird ESR 140.16, released 15 Sep 2026, which includes all of its fixes.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 140.15, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Thunderbird ESR 140.15 still safe?
Not any more. The 2 updates since, up to Thunderbird ESR 140.17, fix 75 security flaws that Thunderbird ESR 140.15 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 14 security flaws, 0 rated critical and 10 high by Mozilla.[1] The 10 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-84639 | High | Uninitialized memory in MIME parsing |
CVE-2026-75874 | High | Sandbox escape in the Remote Settings Client component |
CVE-2026-16365 | High | Privilege escalation in the DOM: Workers component |
CVE-2026-84119 | High | Sandbox escape due to use-after-free in the DOM: Navigation component |
CVE-2026-84120 | High | Use-after-free in the Audio/Video component |
CVE-2026-84121 | High | Sandbox escape due to use-after-free in the DOM: Security component |
CVE-2026-84122 | High | Use-after-free in the Audio/Video component |
CVE-2026-84124 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-84143 | High | Internally found bugs fixed in Thunderbird 155, Thunderbird ESR 153.2 and Thunderbird ESR 140.15 |
CVE-2026-84145 | High | Internally found bugs fixed in Thunderbird 155, Thunderbird ESR 153.2 and Thunderbird ESR 140.15 |
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 140.16 or later.
Older devices
The same day, Mozilla released Thunderbird ESR 153.2 (29 security flaws) for devices that stay on an older version. Each fixes the same holes where they apply.
140.14 vs 140.15
Thunderbird ESR 140.14, released 18 Aug 2026, fixed 31 security flaws. Thunderbird ESR 140.15 fixes 14 security flaws and includes the fixes before it, so going straight to 140.15 gets you both.
- Before: Thunderbird ESR 140.14, 18 Aug 2026, 31 security flaws
- After: Thunderbird ESR 140.16, 15 Sep 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 2026-87”, 1 Sep 2026.