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Should you update to Firefox ESR 140.13?

From Firefox ESR updates, in the HARD wiki · Updated
This article is about one Firefox ESR release. For the latest version, see Firefox ESR updates.

No: install Firefox ESR 140.17 instead. Firefox ESR 140.13 is a security update Mozilla released on 21 Jul 2026 for Mac, Windows and Linux, mostly in workplaces and schools. It fixes 32 security flaws, 2 rated critical and 16 high by Mozilla.[1] None of them is known to be used in attacks.

It has since been replaced by Firefox ESR 140.14, released 18 Aug 2026, which includes all of its fixes.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 140.13, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.

Is Firefox ESR 140.13 still safe?

Not any more. The 4 updates since, up to Firefox ESR 140.17, fix 112 security flaws that Firefox ESR 140.13 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 32 security flaws, 2 rated critical and 16 high by Mozilla.[1] The 18 critical and high ones are:

FlawSeverityWhat Mozilla calls it
CVE-2026-15718CriticalInvalid pointer in the JavaScript: WebAssembly component
CVE-2026-15719CriticalSite isolation issue in the DOM: Navigation component
CVE-2026-16349HighSame-origin policy bypass in the DOM: Navigation component
CVE-2026-16350HighIncorrect boundary conditions in the Audio/Video: cubeb component
CVE-2026-16362HighUse-after-free in the WebRTC: Audio/Video component
CVE-2026-16351HighSandbox escape due to use-after-free in the DOM: Navigation component
CVE-2026-16352HighSandbox escape due to use-after-free in the Disability Access APIs component
CVE-2026-16363HighJIT miscompilation in the JavaScript: WebAssembly component
CVE-2026-16353HighInvalid pointer in the DOM: Bindings (WebIDL) component
CVE-2026-16354HighInformation disclosure in the Graphics: ImageLib component
CVE-2026-16368HighIncorrect boundary conditions in the JavaScript: WebAssembly component
CVE-2026-16369HighInteger overflow in the JavaScript: WebAssembly component
CVE-2026-16355HighJIT miscompilation in the JavaScript Engine: JIT component
CVE-2026-16356HighSandbox escape due to use-after-free in the Disability Access APIs component
CVE-2026-16357HighIncorrect boundary conditions in the Graphics component
CVE-2026-16412HighMemory safety bugs fixed in Firefox ESR 140.13 and Firefox 153
CVE-2026-16360HighMemory safety bugs fixed in Firefox ESR 115.38, Firefox ESR 140.13 and Firefox 153
CVE-2026-16361HighMemory safety bugs fixed in Firefox ESR 115.38 and Firefox ESR 140.13

How to update, and how long it takes

A minute or two. Firefox restarts and reopens your tabs.

  1. 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.
  2. 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 140.14 or later.

Older devices

The same day, Mozilla released Firefox ESR 115.38 (14 security flaws) for devices that stay on an older version. Each fixes the same holes where they apply.

140.12 vs 140.13

Firefox ESR 140.12, released 16 Jun 2026, fixed 29 security flaws. Firefox ESR 140.13 fixes 32 security flaws and includes the fixes before it, so going straight to 140.13 gets you both.

References

  1. ^ Mozilla. “Security Vulnerabilities fixed in Firefox ESR 140.13”, 21 Jul 2026.