The official Firefox 156 release notes state that Mozilla has released Firefox 156.0, which includes faster PDF loading, reduced resource consumption when displaying large JPEG images, a new macOS startup option, and a number of fixes.
Firefox 156.0 was made available to users on the Release channel on September 15, 2026. The built-in PDF viewer is now up to 45% faster to start up, and the browser uses less memory and CPU when showing large JPEG images that have been scaled to fit a page.
The update also has among its features significant security patches.
Firefox 156 Performance, macOS Startup, and Address Bar Changes
The built-in PDF viewer in Firefox is now up to 45% faster to start up, and Mozilla has also improved both memory and CPU usage when Firefox is displaying large JPEG images that have been scaled down to fit a page.
Windows PCs that use ARM64 processors have also been given a different performance adjustment regarding video calls, with WebRTC calls on these devices now having the option of using hardware-based H264 decoding rather than reverting to software decoding.
On macOS Firefox will now start up automatically when the computer is turned on, and users can turn this option on in the Startup section of Firefox Settings.
In France, Germany and Italy Firefox 156 also extends the suggestions in the address bar, with localized Wikipedia results and from time to time sponsored suggestions now being available in these countries, and users are able to turn them off via the Firefox Suggest settings.
Firefox 156 Media, Bookmark, and Network Fixes
A number of fixes deal with issues concerning images, audio, and video. Mozilla has corrected a bug which could cause an image to be replaced by a long string of text when a user dragged the image to a different position within a rich-text editor.
Audio in the form of high-sample-rate FLAC, when stored in MP4 files, now plays properly on sites like Bilibili, and Mozilla has also fixed the problem where subtitles did not show up in a Picture-in-Picture window when they were turned on after the window had already been opened.
A fix was applied to the way bookmarks are handled in the case where a folder and its bookmarks are selected together and then the selection is dragged in the Bookmarks sidebar, as this could have caused the bookmarks to be moved out of their folder.
With regard to Split View, there was another issue in which reversing the two panes could result in the find bar remaining permanently unavailable in the relevant tab, a problem which Mozilla resolved.
The update also looks at cases of websites failing to load involving DNS over HTTPS, since some sites would not load when DNS over HTTPS was enabled and the site itself did not support HTTPS.
Mozilla has corrected a network issue that affected users of Firefox’s built-in VPN, specifically one where Firefox could lose all network connectivity while the VPN was in use. Users of Windows were given a remedy for a problem in which Firefox could prevent an auto-hiding taskbar from appearing.
Firefox 156 Security Patches and Enterprise Policy Changes
The high overall impact rating is given to the security patches included in Firefox 156, which are described in Mozilla Foundation Security Advisory 2026-90 and were announced on September 15.
This advisory details vulnerabilities relating to privilege escalation, use-after-free flaws, incorrect boundary conditions, site isolation, sandbox escapes, mitigation bypasses, and other problems.
Important issues involve CVE-2026-92005, a use-after-free vulnerability in the Audio/Video Web Codecs component, and a number of privilege-escalation vulnerabilities related to Graphics: CanvasWebGL.
The advisory also includes high-impact problems concerning WebExtensions, Graphics, Disability Access APIs, DOM Service Workers, DOM Core and HTML, the Remote Settings Client, WebRender, the HTML parser, XML, SVG, networking, and other components.
The policy changes and fixes mentioned in the Firefox 156 enterprise release notes were received by enterprise administrators, with the majority of them also applying to Firefox ESR 153.3.0 unless Mozilla states otherwise.
FirefoxHome introduced a Widgets setting under the policy that allows the new-tab widgets to be turned on or off using the Enabled option and enables individual widgets to be blocked by means of the Blocked setting.
Together with this, Firefox 156 includes a DisableServiceWorkers option within SitePolicies that permits certain sites to have their registration or use of service workers stopped.
The DisableLaunchOnLogin policy is now applicable on macOS having the launch-on-login feature been added, although Mozilla states that this change does not extend to Firefox ESR.
The fixes implemented by Enterprise cover a number of existing policies. Previously, the actions taken at Firefox startup and within the product to set the default browser had not complied with the DefaultBrowserSettingEnabled settings, and Page Info had not consistently disabled the cookie permission controls which had been set via policy.
Mozilla also addressed the issue of the New Private Window item still being visible in the File menu when DisablePrivateBrowsing was in effect, hidden the Discover extensions button when extension installation is prevented by InstallAddonsPermission, and disabled the new-tab customization controls when the corresponding FirefoxHome preferences are locked.
Firefox 156 Developer Changes and Availability
Web developers will now find that the CSS text-box-trim property makes use of the font metrics of a ::first-line pseudo-element in such cases. Mozilla states that this behaviour is in line with that of the other browsers and the specification.
Firefox 156.0 is now available, with release channel users being the first to receive it on September 15, 2026. Since the security updates have a high impact rating, updating is given top priority.
Enterprise administrators can look at the individual enterprise release notes to see the policy changes, the majority of which also apply to Firefox ESR 153.3.0.
