Should you update to Thunderbird 155?
No: install Thunderbird 159 instead. Thunderbird 155 is a security update Mozilla released on 1 Sep 2026 for Mac, Windows and Linux. It fixes 30 security flaws, 0 rated critical and 13 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Thunderbird 156, released 15 Sep 2026, which includes all of its fixes.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 159 instead. Thunderbird 159 came out on 27 Oct 2026 and includes every fix in Thunderbird 155, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Thunderbird 155 still safe?
Not any more. The 5 updates since, up to Thunderbird 159, fix 411 security flaws that Thunderbird 155 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 30 security flaws, 0 rated critical and 13 high by Mozilla.[1] The 13 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-84639 | High | Uninitialized memory in MIME parsing |
CVE-2026-84118 | High | Use-after-free in the JavaScript: GC 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-2026-84123 | High | Privilege escalation due to use-after-free in the Graphics: WebGPU component |
CVE-2026-84124 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-84125 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-84126 | High | Incorrect boundary conditions in the Layout: Grid component |
CVE-2026-84143 | High | Internally found bugs fixed in Thunderbird 155, Thunderbird ESR 153.2 and Thunderbird ESR 140.15 |
CVE-2026-84144 | High | Internally found bugs fixed in Thunderbird 155 and Thunderbird ESR 153.2 |
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 under the name. It should say 156 or later.
154 vs 155
Thunderbird 154, released 18 Aug 2026, fixed 56 security flaws. Thunderbird 155 fixes 30 security flaws and includes the fixes before it, so going straight to 155 gets you both.
- Before: Thunderbird 154, 18 Aug 2026, 56 security flaws
- After: Thunderbird 156, 15 Sep 2026, 75 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 2026-86”, 1 Sep 2026.