Should you update to Thunderbird 154?
No: install Thunderbird 159 instead. Thunderbird 154 is a security update Mozilla released on 18 Aug 2026 for Mac, Windows and Linux. It fixes 56 security flaws, 0 rated critical and 20 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Thunderbird 155, released 1 Sep 2026, which includes all of its fixes. If you’re on an older version, install that instead.
Should you update?
No: install Thunderbird 159 instead. Thunderbird 159 came out on 27 Oct 2026 and includes every fix in Thunderbird 154, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Thunderbird 154 still safe?
Not any more. The 6 updates since, up to Thunderbird 159, fix 441 security flaws that Thunderbird 154 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 56 security flaws, 0 rated critical and 20 high by Mozilla.[1] The 20 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-75874 | High | Sandbox escape in the Remote Settings Client component |
CVE-2026-74934 | High | Site isolation issue in the Graphics: CanvasWebGL component |
CVE-2026-74935 | High | Privilege escalation in the DOM: Networking component |
CVE-2026-74936 | High | Use-after-free in the JavaScript: WebAssembly component |
CVE-2026-74937 | High | Use-after-free in the JavaScript: GC component |
CVE-2026-74938 | High | Mitigation bypass in the JavaScript: GC component |
CVE-2026-74939 | High | Privilege escalation in the DOM: Navigation component |
CVE-2026-74940 | High | Use-after-free in the Graphics: Text component |
CVE-2026-74941 | High | Privilege escalation in the Graphics: CanvasWebGL component |
CVE-2026-74942 | High | Privilege escalation in the Remote Settings Client component |
CVE-2026-74943 | High | Use-after-free in the Graphics: ImageLib component |
CVE-2026-74944 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-74945 | High | Information disclosure in the Graphics: Text component |
CVE-2026-74946 | High | Privilege escalation due to incorrect boundary conditions in the Graphics: CanvasWebGL component |
CVE-2026-74947 | High | Privilege escalation due to invalid pointer in the Graphics component |
CVE-2026-74948 | High | Information disclosure in the Graphics component |
CVE-2026-74949 | High | Privilege escalation due to use-after-free in the Graphics: Canvas2D component |
CVE-2026-74987 | High | Internally found bugs fixed in Thunderbird ESR 140.14, Thunderbird ESR 153.1 and Thunderbird 154 |
CVE-2026-74988 | High | Internally found bugs fixed in Thunderbird ESR 153.1 and Thunderbird 154 |
CVE-2026-74990 | High | Internally found bugs fixed in Thunderbird ESR 140.14, Thunderbird ESR 153.1 and Thunderbird 154 |
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 under the name. It should say 155 or later.
153 vs 154
Thunderbird 153, released 21 Jul 2026, fixed 61 security flaws. Thunderbird 154 fixes 56 security flaws and includes the fixes before it, so going straight to 154 gets you both.
- Before: Thunderbird 153, 21 Jul 2026, 61 security flaws
- After: Thunderbird 155, 1 Sep 2026, 30 security flaws
- Thunderbird updates, every release and the latest version
- Updates, the latest version of everything HARD tracks
References
- ^ Mozilla. “Mozilla Foundation Security Advisory MFSA 2026-78”, 18 Aug 2026.