Firefox fixed 167 security problems on October 27. Update it from inside Firefox

Mozilla released Firefox 159 on October 27, 2026. It fixes 167 security problems in the browser.

Mozilla rates most of the problems high. That is its second-highest rating.

Thirteen of the fixes close holes in the sandbox. The sandbox is the fence that keeps a website's code away from the rest of your computer.

Firefox ESR also received fixes. ESR is the slower-moving version that schools and companies often use.

The update fixes 167 problems in ESR 153.6 and 80 in the older ESR 115.44.

Mozilla also changed how it counts problems. It now gives one number to each bug, so the counts look larger than before.

The update is free. Firefox installs it the same way on Windows, Mac, and Linux.

This report concerns everyone who uses the Firefox browser on a computer or phone.

The problems are in the browser itself. A page you open can try to use them.

Firefox on phones updates through the app stores.

If you use the ESR version, your update carries the same fixes.

Thunderbird, the free email program from Mozilla, also received an update on the same day.

The advisories say nothing about real attacks with these problems.

Mozilla keeps the flaw details secret until most users have the fix. That is its standard practice.

We do not know how many computers already have the update.

Mozilla does not say whether anyone used the problems against people.