Firefox 156 Ships Scheduler.yield Priority Inheritance, text-box-trim Fixes and a 45% Faster PDF Viewer Startup

Overview

Mozilla released Firefox 156 on September 15, 2026, according to the MDN release notes for developers and the official Firefox release notes. The release is mostly a web-platform conformance and polish update: several JavaScript and DOM behaviors change to match their specifications, a pair of TLS key-exchange groups are dropped from default handshakes, and the built-in PDF viewer starts faster.

What Changed for Web Developers

• Scheduler. Per MDN, Scheduler.yield() now inherits the enclosing task’s priority and abort signal across a synchronously-settling await.
• JavaScript. According to MDN, Promise.try() now resolves the callback’s return value the way Promise.resolve() does, and using declarations can no longer be reassigned, matching const-like semantics.
• CSS. MDN reports that text-box-trim and text-box-edge now trim correctly, including using the font metrics of the ::first-line pseudo-element where applicable. The notes add that the properties still have no effect with line-clamp.
• Web Crypto. MDN says SubtleCrypto.deriveBits() now throws a TypeError if the length parameter is NaN, Infinity, negative, or greater than 2^32-1.
• WebRTC. The RTCPeerConnection() constructor accepts a new alwaysNegotiateDataChannels configuration member that defaults to false, per MDN.
• DOM. Also per MDN, Range.deleteContents() and Range.extractContents() now operate on the DOM tree rather than the flat tree.

Security and Automation Changes

MDN lists the ffdhe2048 and ffdhe3072 finite-field Diffie-Hellman groups as no longer offered by default in TLS handshakes. For WebDriver users, the same notes say the Marionette WebDriver:GetElementTagName command now returns the element’s qualified name, a change described as non-backward-compatible for elements with case-sensitive qualified names such as SVG elements.

Users and Administrators

The Firefox release notes state that PDF viewer startup speeds increased by approximately 45%, and that Windows ARM64 processors now support hardware H264 decoding for WebRTC video calls. macOS users can also set Firefox to open automatically at startup.

For enterprise administrators, the Firefox administrator reference says the release adds a DisableServiceWorkers option under SitePolicies, described as a way to stop specific sites from registering or using service workers. That option applies to Firefox 156 only, the page states, and it notes that Firefox ESR 153 is the current ESR.

Experimental Features

The MDN notes list three web features that remain disabled by default and must be enabled through about:config: scoped custom element registries, named-feature() support queries and the Container Timing API. Scoped custom element registries are enabled by default in Nightly builds.

What We Don’t Know

The sources reviewed do not say when the scoped custom element registry or Container Timing API features will ship to the stable channel. The release notes also do not describe the conditions behind the PDF viewer speedup beyond the approximate 45% figure.

Context

Firefox 156 arrives as browser vendors move to shorter release cycles, a shift The Machine Herald previously reported.