Should you update to Firefox ESR 140.17?
Yes, this week. Firefox ESR 140.17 is a security update Mozilla released on 29 Sep 2026 for Mac, Windows and Linux, mostly in workplaces and schools. It fixes 43 security flaws, 0 rated critical and 32 high by Mozilla.[1] None of them is known to be used in attacks.
It is the newest Firefox ESR 140 release, so it’s the one to have.
Should you update?
Yes, this week. It fixes 43 security flaws, and attackers study each release to work out what it fixed. Every security update closes holes that become public the moment it ships, so the risk of staying on the old version only grows. Waiting a week to see if others hit problems is a common habit; with a security update it’s the wrong trade.
Is Firefox ESR 140.17 safe to install?
Yes. It’s Mozilla’s own security release, and updates rarely go wrong; a backup makes it painless if one does. The one unsafe way to get it is from a link: Mozilla never sends updates by email, text or pop-up, so install it only from Firefox’s own About window, or your IT team.
What it fixes
Mozilla lists 43 security flaws, 0 rated critical and 32 high by Mozilla.[1] The 32 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-100756 | High | Incorrect boundary conditions in the Audio/Video: Playback component |
CVE-2026-100757 | High | Use-after-free in the Widget component |
CVE-2026-100758 | High | Sandbox escape in the DOM: Navigation component |
CVE-2026-100759 | High | Uninitialized memory in the Storage: Quota Manager component |
CVE-2026-100762 | High | Sandbox escape due to use-after-free in the DOM: Content Processes component |
CVE-2026-92035 | High | Sandbox escape due to incorrect boundary conditions in the Graphics component |
CVE-2026-100766 | High | Information disclosure in the Networking: JAR component |
CVE-2026-100767 | High | Use-after-free in the Networking: Cache component |
CVE-2026-100769 | High | Use-after-free in the JavaScript: WebAssembly component |
CVE-2026-100770 | High | Sandbox escape due to use-after-free in the DOM: Content Processes component |
CVE-2026-100771 | High | Undefined behavior in the DOM: Streams component |
CVE-2026-100772 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-100773 | High | Use-after-free in the Storage: IndexedDB component |
CVE-2026-100774 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-100775 | High | Sandbox escape in the Graphics component |
CVE-2026-100776 | High | Use-after-free in the JavaScript: WebAssembly component |
CVE-2026-100777 | High | Use-after-free in the Graphics: Canvas2D component |
CVE-2026-100778 | High | Sandbox escape due to use-after-free in the DOM: Core & HTML component |
CVE-2026-100779 | High | Use-after-free in the XSLT component |
CVE-2026-100780 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-100781 | High | Sandbox escape due to incorrect boundary conditions in the Graphics: WebRender component |
CVE-2026-100782 | High | Privilege escalation due to incorrect boundary conditions in the Graphics component |
CVE-2026-100783 | High | Uninitialized memory in the Audio/Video component |
CVE-2026-100784 | High | Use-after-free in the Layout: Text and Fonts component |
CVE-2026-100785 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-100786 | High | Sandbox escape due to use-after-free in the Graphics component |
CVE-2026-100788 | High | Invalid pointer in the JavaScript: WebAssembly component |
CVE-2026-100789 | High | Use-after-free in the Graphics: Canvas2D component |
CVE-2026-100790 | High | Use-after-free in the XSLT component |
CVE-2026-100791 | High | Use-after-free in the DOM: Core & HTML component |
| and 2 more | ||
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 140.17 or later.
Older devices
The same day, Mozilla released Firefox ESR 153.4 (62 security flaws) and Firefox ESR 115.42 (31 security flaws) for devices that stay on an older version. Each fixes the same holes where they apply.
140.16 vs 140.17
Firefox ESR 140.16, released 15 Sep 2026, fixed 28 security flaws. Firefox ESR 140.17 fixes 43 security flaws and includes the fixes before it, so going straight to 140.17 gets you both.
- Before: Firefox ESR 140.16, 15 Sep 2026, 28 security flaws
- After: none yet. This is the newest.
- 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 140.17”, 29 Sep 2026.