Firefox 158 is out for Windows, macOS, Linux, and Android. This version brings improved PDF support, the ability to fill forms with the help of “AI,” as well as fixes for over 75 security vulnerabilities. Updates are also available for Firefox ESR versions.
Firefox 159’s release is scheduled for October 27th, 2026.
What’s new in Firefox 158?
Following the new Nova design introduced in Firefox 157, now a few technical boosts are being rolled out. The AI-based Smart Windows feature is designed to help users fill in forms more quickly. It’s being rolled out gradually, so it won’t be available to all users straight away.
Another improvement involves the saving of web pages as PDFs. Firefox now generates more accessible PDF files (i.e., fewer barriers), and a number of other shortcomings in PDF rendering have been fixed.
Firefox now supports the JPEG XL image format, which has been running as a Firefox Labs experiment since June. Image compression with JPEG XL is more efficient than with older formats like JPEG, PNG, and WebP, all while maintaining comparable image quality.
Security flaws fixed in Firefox 158
In the security advisory for Firefox 158, Mozilla lists 77 security flaws that have been fixed, 37 of which were discovered and reported by external security researchers (OpenAI’s Amy Burnett stands out with 9 reports).
Mozilla classifies 30 vulnerabilities as “high risk,” 33 as “medium risk,” and 14 as “low risk.” Among the fixed security vulnerabilities are 24 use-after-free (UAF) flaws, some of which can be exploited to run malicious code. Eight vulnerabilities can be exploited to escape the browser’s sandbox.
Firefox ESR updates
In addition to Firefox 158.0, ESR versions 153.5.0 and 115.43.0 are also now available. However, the latter is only available for Windows 7 and 8.1 and macOS 10.12 to 10.14, and will remain so until March 2027.
In the ESR versions, Mozilla has fixed the same aforementioned flaws that were also present in the code of these older browser generations. That amounts to 61 fixes in Firefox 153.5.0 and 17 in Firefox 115.43. (Firefox ESR 140 received its final update on September 29th.)