Should you update to Firefox ESR 153.5.1?
No: install Firefox ESR 153.6 instead. Firefox ESR 153.5.1 is a security update Mozilla released on 20 Oct 2026 for Mac, Windows and Linux, mostly in workplaces and schools. It fixes 1 security flaw, 0 rated critical and 1 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Firefox ESR 153.6, released 27 Oct 2026, which includes all of its fixes.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 153.5.1, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Firefox ESR 153.5.1 still safe?
Not any more. Firefox ESR 153.6 fixes 145 security flaws that Firefox ESR 153.5.1 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 1 security flaw, 0 rated critical and 1 high by Mozilla.[1] The critical or high one is:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-113470 | High | Cryptographic issue in the Security Block-lists, Allow-lists, and other State component |
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 153.6 or later.
Older devices
The same day, Mozilla released Firefox ESR 115.43.1 (1 security flaw) for devices that stay on an older version. Each fixes the same holes where they apply.
153.5 vs 153.5.1
Firefox ESR 153.5, released 13 Oct 2026, fixed 61 security flaws. Firefox ESR 153.5.1 fixes 1 security flaw and includes the fixes before it, so going straight to 153.5.1 gets you both.
- Before: Firefox ESR 153.5, 13 Oct 2026, 61 security flaws
- After: Firefox ESR 153.6, 27 Oct 2026, 145 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 153.5.1”, 20 Oct 2026.