Should you update to Thunderbird ESR 153.5?
No: install Thunderbird ESR 153.6 instead. Thunderbird ESR 153.5 is a security update Mozilla released on 13 Oct 2026 for Mac, Windows and Linux. It fixes 62 security flaws, 0 rated critical and 29 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Thunderbird ESR 153.5.1, released 20 Oct 2026, which includes all of its fixes.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, 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 still safe?
Not any more. The 2 updates since, up to Thunderbird ESR 153.6, fix 158 security flaws that Thunderbird ESR 153.5 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 62 security flaws, 0 rated critical and 29 high by Mozilla.[1] The 29 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-109704 | High | Privilege escalation in the Application Update component |
CVE-2026-109705 | High | Sandbox escape, site isolation issue due to incorrect boundary conditions, use-after-free in the Networking component |
CVE-2026-109706 | High | Use-after-free in the Storage: IndexedDB component |
CVE-2026-109707 | High | Sandbox escape due to use-after-free in the Networking component |
CVE-2026-109708 | High | Sandbox escape due to incorrect boundary conditions in the Networking component |
CVE-2026-109709 | High | Information disclosure, privilege escalation in the Security: Process Sandboxing component |
CVE-2026-109710 | High | Use-after-free in the Graphics: Text component |
CVE-2026-109711 | High | Use-after-free in the Graphics: Canvas2D component |
CVE-2026-109712 | High | Sandbox escape due to uninitialized memory in the Graphics component |
CVE-2026-109713 | High | Use-after-free in the Crash Reporting component |
CVE-2026-109715 | High | Incorrect boundary conditions in the JavaScript Engine component |
CVE-2026-109716 | High | Incorrect boundary conditions in the JavaScript Engine component |
CVE-2026-109717 | High | Use-after-free in the DOM: Bindings (WebIDL) component |
CVE-2026-109718 | High | Use-after-free in the XML component |
CVE-2026-109719 | High | Race condition, use-after-free in the Graphics: CanvasWebGL component |
CVE-2026-109720 | High | JIT miscompilation in the JavaScript: WebAssembly component |
CVE-2026-109721 | High | Incorrect boundary conditions in the JavaScript Engine: JIT component |
CVE-2026-109722 | High | JIT miscompilation in the JavaScript Engine: JIT component |
CVE-2026-109723 | High | Integer overflow in the Layout: Text and Fonts component |
CVE-2026-109724 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-109725 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-109726 | High | Use-after-free in the SVG component |
CVE-2026-109727 | High | Use-after-free in the Audio/Video component |
CVE-2026-109728 | High | Use-after-free in the DOM: Animation component |
CVE-2026-109729 | High | Use-after-free in the JavaScript Engine: JIT component |
CVE-2026-109730 | High | Integer overflow in the Graphics component |
CVE-2026-109731 | High | Incorrect boundary conditions in the Graphics: Canvas2D component |
CVE-2026-109732 | High | Use-after-free in the DOM: Streams component |
CVE-2026-109733 | High | JIT miscompilation in the JavaScript Engine: JIT 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 153.5.1 or later.
153.4 vs 153.5
Thunderbird ESR 153.4, released 30 Sep 2026, fixed 63 security flaws. Thunderbird ESR 153.5 fixes 62 security flaws and includes the fixes before it, so going straight to 153.5 gets you both.
- Before: Thunderbird ESR 153.4, 30 Sep 2026, 63 security flaws
- After: Thunderbird ESR 153.5.1, 20 Oct 2026, 4 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 2026-110”, 13 Oct 2026.