Should you update to Firefox ESR 115.40?
No: install Firefox ESR 115.44 instead. Firefox ESR 115.40 is a security update Mozilla released on 1 Sep 2026 for Mac, Windows and Linux, mostly in workplaces and schools. It fixes 6 security flaws, 0 rated critical and 5 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Firefox ESR 115.41, released 15 Sep 2026, which includes all of its fixes.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 Firefox ESR 115.44 instead. Firefox ESR 115.44 came out on 27 Oct 2026 and includes every fix in Firefox ESR 115.40, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Firefox ESR 115.40 still safe?
Not any more. The 5 updates since, up to Firefox ESR 115.44, fix 150 security flaws that Firefox ESR 115.40 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 6 security flaws, 0 rated critical and 5 high by Mozilla.[1] The 5 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-75874 | High | Sandbox escape in the Remote Settings Client component |
CVE-2026-84119 | 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-2026-84145 | High | Internally found bugs fixed in Firefox 155, Firefox ESR 153.2, Firefox ESR 140.15 and Firefox ESR 115.40 |
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 115.41 or later.
Older devices
The same day, Mozilla released Firefox ESR 153.2 (24 security flaws) and Firefox ESR 140.15 (11 security flaws) for devices that stay on an older version. Each fixes the same holes where they apply.
115.39 vs 115.40
Firefox ESR 115.39, released 18 Aug 2026, fixed 13 security flaws. Firefox ESR 115.40 fixes 6 security flaws and includes the fixes before it, so going straight to 115.40 gets you both.
- Before: Firefox ESR 115.39, 18 Aug 2026, 13 security flaws
- After: Firefox ESR 115.41, 15 Sep 2026, 21 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 115.40”, 1 Sep 2026.