Should you update to Firefox ESR 140.12?
No: install Firefox ESR 140.17 instead. Firefox ESR 140.12 is a security update Mozilla released on 16 Jun 2026 for Mac, Windows and Linux, mostly in workplaces and schools. It fixes 29 security flaws, 0 rated critical and 12 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Firefox ESR 140.13, released 21 Jul 2026, which includes all of its fixes. If you’re on an older version, install that instead.
Should you update?
No: install Firefox ESR 140.17 instead. Firefox ESR 140.17 came out on 29 Sep 2026 and includes every fix in Firefox ESR 140.12, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Firefox ESR 140.12 still safe?
Not any more. The 5 updates since, up to Firefox ESR 140.17, fix 144 security flaws that Firefox ESR 140.12 doesn’t, and each became public the day its fix shipped. Installing it is safe: get it only from Firefox’s own About window, or your IT team, never from a link in an email, text or pop-up.
What it fixes
Mozilla lists 29 security flaws, 0 rated critical and 12 high by Mozilla.[1] The 12 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-12289 | High | Privilege escalation in the Graphics: WebRender component |
CVE-2026-12290 | High | Memory safety bug fixed in Firefox ESR 140.12 |
CVE-2026-12291 | High | Use-after-free in the Networking: HTTP component |
CVE-2026-12292 | High | Incorrect boundary conditions in the Web Audio component |
CVE-2026-12294 | High | Sandbox escape in the DOM: Workers component |
CVE-2026-12295 | High | Sandbox escape in the DOM: Navigation component |
CVE-2026-12298 | High | Memory safety bug fixed in Firefox ESR 140.12 |
CVE-2026-12296 | High | Sandbox escape in the Security: Process Sandboxing component |
CVE-2026-12297 | High | Sandbox escape due to incorrect boundary conditions in the Networking component |
CVE-2026-12299 | High | JIT miscompilation in the DOM: Core & HTML component |
CVE-2026-12329 | High | Memory safety bug fixed in Firefox ESR 140.12 |
CVE-2026-12328 | High | Memory safety bugs fixed in Firefox ESR 115.37, Firefox ESR 140.12, Thunderbird ESR 140.12, Firefox 152 and Thunderbird 152 |
How to update, and how long it takes
A minute or two. Firefox restarts and reopens your tabs.
- Open Firefox’s About window.Open the ☰ menu → Help → About Firefox. If your workplace or school manages Firefox, its IT team may install ESR updates for you.
- Click Restart to update Firefox.Firefox reopens your windows and tabs. Until it restarts, the old, unfixed version keeps running.
To check it worked: The ☰ menu → Help → About Firefox shows the version, ending in esr. It should say 140.13 or later.
Older devices
The same day, Mozilla released Firefox ESR 115.37 (11 security flaws) for devices that stay on an older version. Each fixes the same holes where they apply.
Before and after
- After: Firefox ESR 140.13, 21 Jul 2026, 32 security flaws
- Firefox ESR updates, every release and the latest version
- Updates, the latest version of everything HARD tracks
References
- ^ Mozilla. “Security Vulnerabilities fixed in Firefox ESR 140.12”, 16 Jun 2026.