Should you update to Thunderbird ESR 153.3?
No: install Thunderbird ESR 153.6 instead. Thunderbird ESR 153.3 is a security update Mozilla released on 16 Sep 2026 for Mac, Windows and Linux. It fixes 66 security flaws, 0 rated critical and 25 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Thunderbird ESR 153.4, released 30 Sep 2026, which includes all of its fixes.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 153.6 instead. Thunderbird ESR 153.6 came out on 27 Oct 2026 and includes every fix in Thunderbird ESR 153.3, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Thunderbird ESR 153.3 still safe?
Not any more. The 4 updates since, up to Thunderbird ESR 153.6, fix 282 security flaws that Thunderbird ESR 153.3 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 66 security flaws, 0 rated critical and 25 high by Mozilla.[1] The 25 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-92005 | High | Use-after-free in the Audio/Video: Web Codecs component |
CVE-2026-92006 | High | Privilege escalation due to incorrect boundary conditions in the Graphics: CanvasWebGL component |
CVE-2026-92007 | High | Privilege escalation due to incorrect boundary conditions in the Graphics: CanvasWebGL component |
CVE-2026-92008 | High | Privilege escalation due to incorrect boundary conditions in the Graphics: CanvasWebGL component |
CVE-2026-92009 | High | Privilege escalation due to incorrect boundary conditions in the Graphics: CanvasWebGL component |
CVE-2026-92010 | High | Privilege escalation due to incorrect boundary conditions in the Graphics: CanvasWebGL component |
CVE-2026-92011 | High | Privilege escalation due to incorrect boundary conditions in the Graphics: CanvasWebGL component |
CVE-2026-92012 | High | Privilege escalation due to incorrect boundary conditions in the Graphics: CanvasWebGL component |
CVE-2026-92013 | High | Privilege escalation due to incorrect boundary conditions in the Graphics: CanvasWebGL component |
CVE-2026-92015 | High | Privilege escalation in the WebExtensions component |
CVE-2026-92035 | High | Sandbox escape due to incorrect boundary conditions in the Graphics component |
CVE-2026-92016 | High | Use-after-free in the Disability Access APIs component |
CVE-2026-92017 | High | Privilege escalation in the DOM: Service Workers component |
CVE-2026-92018 | High | Sandbox escape in the DOM: Core & HTML component |
CVE-2026-92019 | High | Mitigation bypass in the Remote Settings Client component |
CVE-2026-92020 | High | Privilege escalation due to incorrect boundary conditions in the Graphics: WebRender component |
CVE-2026-92022 | High | Use-after-free in the DOM: HTML Parser component |
CVE-2026-92023 | High | Use-after-free in the XML component |
CVE-2026-92024 | High | Use-after-free in the SVG component |
CVE-2026-92025 | High | Use-after-free in the DOM: Navigation component |
CVE-2026-92026 | High | Use-after-free in the Networking component |
CVE-2026-92027 | High | Use-after-free in the DOM: Streams component |
CVE-2026-92028 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-92029 | High | Use-after-free in the SVG component |
CVE-2026-92038 | High | Mitigation bypass in the Remote Settings Client 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 153.4 or later.
153.2 vs 153.3
Thunderbird ESR 153.2, released 1 Sep 2026, fixed 29 security flaws. Thunderbird ESR 153.3 fixes 66 security flaws and includes the fixes before it, so going straight to 153.3 gets you both.
- Before: Thunderbird ESR 153.2, 1 Sep 2026, 29 security flaws
- After: Thunderbird ESR 153.4, 30 Sep 2026, 63 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 2026-96”, 16 Sep 2026.