Should you update to Firefox ESR 153.5?
No: install Firefox ESR 153.6 instead. Firefox ESR 153.5 is a security update Mozilla released on 13 Oct 2026 for Mac, Windows and Linux, mostly in workplaces and schools. It fixes 61 security flaws, 0 rated critical and 29 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Firefox ESR 153.5.1, released 20 Oct 2026, which includes all of its fixes.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, 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 still safe?
Not any more. The 2 updates since, up to Firefox ESR 153.6, fix 146 security flaws that Firefox ESR 153.5 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 61 security flaws, 0 rated critical and 29 high by Mozilla.[1] The 29 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-109704 | High | Privilege escalation in the Application Update component |
CVE-2026-109705 | High | Sandbox escape, site isolation issue due to incorrect boundary conditions, use-after-free in the Networking component |
CVE-2026-109706 | High | Use-after-free in the Storage: IndexedDB component |
CVE-2026-109707 | High | Sandbox escape due to use-after-free in the Networking component |
CVE-2026-109708 | High | Sandbox escape due to incorrect boundary conditions in the Networking component |
CVE-2026-109709 | High | Information disclosure, privilege escalation in the Security: Process Sandboxing component |
CVE-2026-109710 | High | Use-after-free in the Graphics: Text component |
CVE-2026-109711 | High | Use-after-free in the Graphics: Canvas2D component |
CVE-2026-109712 | High | Sandbox escape due to uninitialized memory in the Graphics component |
CVE-2026-109713 | High | Use-after-free in the Crash Reporting component |
CVE-2026-109715 | High | Incorrect boundary conditions in the JavaScript Engine component |
CVE-2026-109716 | High | Incorrect boundary conditions in the JavaScript Engine component |
CVE-2026-109717 | High | Use-after-free in the DOM: Bindings (WebIDL) component |
CVE-2026-109718 | High | Use-after-free in the XML component |
CVE-2026-109719 | High | Race condition, use-after-free in the Graphics: CanvasWebGL component |
CVE-2026-109720 | High | JIT miscompilation in the JavaScript: WebAssembly component |
CVE-2026-109721 | High | Incorrect boundary conditions in the JavaScript Engine: JIT component |
CVE-2026-109722 | High | JIT miscompilation in the JavaScript Engine: JIT component |
CVE-2026-109723 | High | Integer overflow in the Layout: Text and Fonts component |
CVE-2026-109724 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-109725 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-109726 | High | Use-after-free in the SVG component |
CVE-2026-109727 | High | Use-after-free in the Audio/Video component |
CVE-2026-109728 | High | Use-after-free in the DOM: Animation component |
CVE-2026-109729 | High | Use-after-free in the JavaScript Engine: JIT component |
CVE-2026-109730 | High | Integer overflow in the Graphics component |
CVE-2026-109731 | High | Incorrect boundary conditions in the Graphics: Canvas2D component |
CVE-2026-109732 | High | Use-after-free in the DOM: Streams component |
CVE-2026-109733 | High | JIT miscompilation in the JavaScript Engine: JIT 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 153.5.1 or later.
Older devices
The same day, Mozilla released Firefox ESR 115.43 (17 security flaws) for devices that stay on an older version. Each fixes the same holes where they apply.
153.4 vs 153.5
Firefox ESR 153.4, released 29 Sep 2026, fixed 62 security flaws. Firefox ESR 153.5 fixes 61 security flaws and includes the fixes before it, so going straight to 153.5 gets you both.
- Before: Firefox ESR 153.4, 29 Sep 2026, 62 security flaws
- After: Firefox ESR 153.5.1, 20 Oct 2026, 1 security flaw
- 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”, 13 Oct 2026.