Should you update to Firefox 155?
No: install Firefox 159.0.1 instead. Firefox 155 is a security update Mozilla released on 1 Sep 2026 for Mac, Windows and Linux. It fixes 29 security flaws, 0 rated critical and 13 high by Mozilla.[1] None of them is known to be used in attacks.
It has since been replaced by Firefox 156, released 15 Sep 2026, which includes all of its fixes.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 159.0.1 instead. Firefox 159.0.1 came out on 3 Nov 2026 and includes every fix in Firefox 155, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is Firefox 155 still safe?
Not any more. The 7 updates since, up to Firefox 159.0.1, fix 398 security flaws that Firefox 155 doesn’t, and each became public the day its fix shipped. Installing it is safe: get it only from Firefox’s own About window (☰ menu → Help → About Firefox), never from a link in an email, text or pop-up.
What it fixes
Mozilla lists 29 security flaws, 0 rated critical and 13 high by Mozilla.[1] The 13 critical and high ones are:
| Flaw | Severity | What Mozilla calls it |
|---|---|---|
CVE-2026-84117 | High | Privilege escalation in Firefox for Android |
CVE-2026-84118 | High | Use-after-free in the JavaScript: GC 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-2026-84122 | High | Use-after-free in the Audio/Video component |
CVE-2026-84123 | High | Privilege escalation due to use-after-free in the Graphics: WebGPU component |
CVE-2026-84124 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-84125 | High | Use-after-free in the DOM: Core & HTML component |
CVE-2026-84126 | High | Incorrect boundary conditions in the Layout: Grid component |
CVE-2026-84143 | High | Internally found bugs fixed in Firefox 155, Firefox ESR 153.2 and Firefox ESR 140.15 |
CVE-2026-84144 | High | Internally found bugs fixed in Firefox 155 and Firefox ESR 153.2 |
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. Firefox checks for the update and downloads it.
- Click Restart to update Firefox.Firefox reopens your windows and tabs. Until it restarts, the old, unfixed version keeps running.
- Leave automatic updates on.In Settings → General → Firefox Updates, keep “Automatically install updates” selected.
To check it worked: The ☰ menu → Help → About Firefox shows the version. It should say 156 or later.
154 vs 155
Firefox 154, released 18 Aug 2026, fixed 58 security flaws. Firefox 155 fixes 29 security flaws and includes the fixes before it, so going straight to 155 gets you both.
- Before: Firefox 154, 18 Aug 2026, 58 security flaws
- After: Firefox 156, 15 Sep 2026, 73 security flaws
- Firefox updates, every release and the latest version
- Updates, the latest version of everything HARD tracks
References
- ^ Mozilla. “Security Vulnerabilities fixed in Firefox 155”, 1 Sep 2026.