Mozilla has released Firefox 158 for Windows, macOS, Linux and Android, bringing a new AI assisted form filling feature, improved PDF support, JPEG XL compatibility and fixes for 77 security vulnerabilities.
The update includes several important security patches, with Mozilla classifying 30 of the fixed flaws as high risk. Because some of the vulnerabilities could potentially allow malicious code execution or sandbox escapes, updating Firefox should be a priority if you are still running an older version.
Mozilla has also released corresponding updates for its Extended Support Release editions.
Firefox 158 Introduces AI Assisted Form Filling
One of the main additions in Firefox 158 is an AI based feature called Smart Windows.
The feature is designed to help you complete web forms more quickly by automatically assisting with form filling.
Mozilla is introducing Smart Windows gradually, so installing Firefox 158 does not guarantee that the feature will immediately appear on your system.
| Firefox 158 change | Details |
|---|---|
| AI feature | Smart Windows form filling |
| Security fixes | 77 vulnerabilities |
| High risk flaws | 30 |
| Medium risk flaws | 33 |
| Low risk flaws | 14 |
| Use after free flaws | 24 |
| Sandbox escape flaws | 8 |
| New image format | JPEG XL |
| Next Firefox release | October 27, 2026 |
The gradual rollout approach means some users may receive the new AI functionality later than others even when they are on the same browser version.
JPEG XL Support Moves Beyond Firefox Labs
Firefox 158 also introduces full support for JPEG XL.
The image format had been available through Firefox Labs as an experimental feature since June, but it is now moving into the regular browser.
JPEG XL is designed to offer more efficient image compression than older formats such as JPEG, PNG and WebP while maintaining comparable visual quality.
More efficient compression can reduce image file sizes, which may help websites load faster and lower bandwidth usage when the format is supported by both websites and browsers.
PDF Saving and Rendering Get Improvements
Mozilla has also made changes to Firefox's PDF handling.
Web pages saved as PDF files should now produce more accessible documents with fewer barriers for people using accessibility tools.
The browser also fixes several other issues affecting PDF rendering.
These changes are less visible than the new AI feature, but they should improve Firefox for people who regularly save web content as PDFs or view documents directly in the browser.
Mozilla Fixes 77 Security Vulnerabilities
Security is one of the most significant parts of the Firefox 158 release.
Mozilla lists 77 fixed vulnerabilities in the update, including 37 issues reported by external security researchers.
Of the total, 30 are classified as high risk, 33 as medium risk and 14 as low risk.
The update fixes 24 use after free vulnerabilities. Some of these flaws may be exploitable to execute malicious code.
Mozilla also patched eight vulnerabilities that could potentially allow attackers to escape Firefox's browser sandbox.
The sandbox is designed to isolate web content from the rest of the operating system, so vulnerabilities capable of bypassing that protection can be particularly important.
Firefox ESR Versions Receive Security Updates
Mozilla has also updated its Extended Support Release branches.
Firefox ESR 153.5.0 includes 61 fixes for vulnerabilities that were also present in that older codebase.
Firefox ESR 115.43.0 receives 17 fixes.
The older ESR 115 branch is limited to Windows 7, Windows 8.1 and macOS versions 10.12 through 10.14. Mozilla plans to continue supporting that release until March 2027.
Firefox ESR 140 received its final update on September 29.
Firefox 158 is therefore a substantial update even beyond its new AI features. The combination of 77 security fixes, JPEG XL support and PDF improvements makes the release relevant even if you have no interest in AI assisted form filling.
Mozilla is currently planning to release Firefox 159 on October 27, 2026.
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