Should you update to Thunderbird ESR 153.5.1?
No: install Thunderbird ESR 153.6 instead. Thunderbird ESR 153.5.1 is a security update Mozilla released on 20 Oct 2026 for Mac, Windows and Linux. It fixes 4 security flaws, 0 rated critical and 4 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Thunderbird ESR 153.6, released 27 Oct 2026, which includes all of its fixes. If you’re on an older version, install that instead.
Should you update?
No: install Thunderbird ESR 153.6 instead. Thunderbird ESR 153.6 came out on 27 Oct 2026 and includes every fix in Thunderbird ESR 153.5.1, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Thunderbird ESR 153.5.1 still safe?
Not any more. Thunderbird ESR 153.6 fixes 154 security flaws that Thunderbird ESR 153.5.1 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 4 security flaws, 0 rated critical and 4 high by Mozilla.[1] The 4 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-113866 | High | Account password sent unencrypted during Exchange account setup |
CVE-2026-113867 | High | Stanza injection in XMPP chat |
CVE-2026-113868 | High | Uninitialized memory disclosure and out-of-bounds read in POP3 and SMTP NTLM authentication |
CVE-2026-113470 | High | Cryptographic issue in the Security Block-lists, Allow-lists, and other State component |
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 153.6 or later.
153.5 vs 153.5.1
Thunderbird ESR 153.5, released 13 Oct 2026, fixed 62 security flaws. Thunderbird ESR 153.5.1 fixes 4 security flaws and includes the fixes before it, so going straight to 153.5.1 gets you both.
- Before: Thunderbird ESR 153.5, 13 Oct 2026, 62 security flaws
- After: Thunderbird ESR 153.6, 27 Oct 2026, 154 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 2026-115”, 20 Oct 2026.