Firefox 155 patches 30 security flaws, adds sortable tab groups
The newest major release of Firefox 155 for Windows, macOS, Linux, and Android brings a number of improvements, including support for Nintendo Switch Pro controllers and sortable tab groups. Mozilla has also patched over 30 security vulnerabilities, and updates are also available for the ESR versions of Firefox for older systems.
The release of the next major version, Firefox 156, is already scheduled for September 15th, with time for a bug-fix update on September 8th.
What’s new in Firefox 155?
The “AI” feature known as “Smart Window” (currently still labeled as a beta version) now makes it easier to revisit topics you’ve already covered. Questions are intended to be answered more quickly, complete with source references. It’s being rolled out gradually.
The isolated tab environments introduced with Firefox 153 can now be sorted according to user preferences in settings. Tab environments isolate cookies from one another, allowing you to log in to the same website with different accounts across different tabs. This means you can, for example, actively use multiple Gmail inboxes at once.
Another small improvement involves the display in the address bar that shows how many trackers were blocked on the current page. Firefox now allows this display to be disabled in the settings without turning off tracker blocking altogether. This feature is being rolled out gradually.
The free built-in VPN feature, which was introduced back in Firefox 149, continues to offer the expanded selection of 32 virtual locations, but now that August is over, the unlimited data usage has been ended and is now capped at 50GB per month again. The free VPN is now also rolling out on Firefox for Android, with iOS set to follow soon.
Other improvements include: the ability to scale up small PDF files to fit paper size when printing, and support for Nintendo Switch Pro controllers when playing browser-based online games on Windows. Since Firefox 154, there’s also been support for game streaming with Nvidia GeForce Now.
Security flaws fixed in Firefox 155
In the security advisory for Firefox 155, Mozilla lists 29 security vulnerabilities that have been fixed, 25 of which were discovered and reported by external security researchers. Mozilla classifies 10 of these vulnerabilities as high risk, including several use-after-free (UAF) flaws that could allow malicious code to escape the browser’s sandbox.
The last four entries in the security advisory (CVE-2026-84142 to CVE-2026-84145) summarize an unspecified number of internally discovered security flaws—most of which are classified as high risk—resulting from programming errors in memory management. The vulnerabilities are grouped according to which program versions are affected.
Tip: Whether you keep your browser up to date, you need proper antivirus protections if you want your PC to remain secure and private. Check out our picks for the best antivirus software for Windows as well as best VPN services to stay ahead of security problems.
Firefox ESR browser updates
Alongside Firefox 155.0, the ESR versions 153.2.0, 140.15.0, and 115.40.0 are also available. The latter is only available for Windows 7 and 8.1, and macOS 10.12 to 10.14, and will remain so at least until March 2027.
In the ESR versions, Mozilla has fixed the above-mentioned vulnerabilities that were also present in the code of those previous browser generations. This amounts to more than 24 vulnerabilities fixed in Firefox 153.2.0, at least 11 in Firefox 140.15, and at least six in Firefox 115.40.
Firefox ESR 140 will receive a final update to version 140.17 at the end of September. This generation will then be replaced by Firefox ESR 153.
