Should you update to Firefox ESR 153.1?
No: install Firefox ESR 153.6 instead. Firefox ESR 153.1 is a security update Mozilla released on 18 Aug 2026 for Mac, Windows and Linux, mostly in workplaces and schools. It fixes 52 security flaws, 0 rated critical and 15 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Firefox ESR 153.2, released 1 Sep 2026, which includes all of its fixes.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 Firefox ESR 153.6 instead. Firefox ESR 153.6 came out on 27 Oct 2026 and includes every fix in Firefox ESR 153.1, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Firefox ESR 153.1 still safe?
Not any more. The 6 updates since, up to Firefox ESR 153.6, fix 355 security flaws that Firefox ESR 153.1 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 52 security flaws, 0 rated critical and 15 high by Mozilla.[1] The 15 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
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-2026-74941 | High | Privilege escalation in the Graphics: CanvasWebGL 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-2026-74947 | High | Privilege escalation due to invalid pointer in the Graphics component |
CVE-2026-74948 | High | Information disclosure in the Graphics component |
CVE-2026-74987 | High | Internally found bugs fixed in Firefox ESR 140.14, Firefox ESR 153.1 and Firefox 154 |
CVE-2026-74988 | High | Internally found bugs fixed in Firefox ESR 153.1 and Firefox 154 |
CVE-2026-74990 | High | Internally found bugs fixed in Firefox ESR 115.39, Firefox ESR 140.14, Firefox ESR 153.1 and Firefox 154 |
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 153.2 or later.
Older devices
The same day, Mozilla released Firefox ESR 140.14 (31 security flaws) and Firefox ESR 115.39 (13 security flaws) for devices that stay on an older version. Each fixes the same holes where they apply.
Before and after
- After: Firefox ESR 153.2, 1 Sep 2026, 24 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 153.1”, 18 Aug 2026.