Firefox 18 release notes for developers - Mozilla

Changes for Web developers

The reversed attribute of the <ol> element is now supported (Firefox bug 601912).
The crossorigin attribute of the <link> element is now supported (Firefox bug 786564).
The allowfullscreen attribute of the <iframe> has been implemented and its prefixed predecessor mozallowfullscreen is now deprecated.

The min-width and min-height now uses the auto keyword as initial value (This has an effect only on flex items as it resolves to 0, the previous initial value, for other items). (Firefox bug 763689)
The cascade has been updated: now author !important rules override CSS animations. (Firefox bug 783714)
The background shorthand property now recognizes CSS3 background-size property specified inside. (Firefox bug 570326)
Initial support for the CSS Flexbox Module has been landed. It is disabled by default but can be enabled by setting layout.css.flexbox.enabled to true (Firefox bug 666041).

navigator.mozPay has been landed. (Firefox bug 767818)
window.devicePixelRatio has been landed. (Firefox bug 564815)
The OS X backend for window.navigator.battery has been implemented. (Firefox bug 696045)
MozBlobBuilder is removed. Developers need to use Blob constructor for creating a Blob object. (Firefox bug 744907)
The visibilitychange event and the Page Visibility API has been unprefixed (Firefox bug 812086).
TextDecoder and TextEncoder have been added. Note that the implementation and spec of these evolved and have been changed in Firefox 19 (Firefox bug 764234).
HTMLMediaElement.src has been separate in two properties: the standard src property, dealing with strings, and the prefixed mozSrcObject property, dealing with media streams (Firefox bug 792665).
Support for transferable objects has been added.
The Screen.lockOrientation() method now supports an Array of strings as argument (Firefox bug 784549.

Harmony's (ECMAScript 2015) Direct Proxies have been landed (Firefox bug 703537). Warning: the implementation contains a couple of known bugs, missing features and misalignments with the current state of the spec. Do not rely on it for production code.
The ECMAScript 2015 contains() method is now implemented on strings. This is unfortunately not compatible with Mootools 1.2, which expects different behavior from contains() on strings but does not ensure it. Newer versions of Mootools fix this issue; sites should upgrade their Mootools version to something newer than 1.2.

The prefixed version of the EXT_texture_filter_anisotropic WebGL extension, "MOZ_EXT_texture_filter_anisotropic" has been removed (Firefox bug 790946).

Quality factors ("q-values") are now clamped to 2 digits (e.g., in HTTP Accept-Language headers) (Firefox bug 672448).
The ALLOW-FROM syntax of the X-FRAME-OPTIONS HTTP Response header is now supported (Firefox bug 690168).

nsIStreamListener: The 4th parameter (aOffset) of onDataAvailable() method changes to unsigned long long. (Firefox bug 784912)
nsIUploadChannel: setUploadStream() supports over 2GB content-length (Firefox bug 790617)
nsIEditor: addEditorObserver() has been removed, use setEditorObserver() instead, removeEditorObserver() no longer takes a nsIEditorObserver parameter (Firefox bug 785091)
nsIHttpProtocolHandler: http-on-modify-request observers are no longer guaranteed to be called synchronously during nsIChannel.asyncOpen(). For observers that need to be called during asyncOpen(), the new http-on-opening-request observer topic has been added. See (Firefox bug 800799)
nsIProtocolProxyService: The resolve method has been removed. Now, only the asyncResolve method can be used. See (Firefox bug 769764)

The following interfaces have been removed.

This page was last modified on ⁨Oct 16, 2025⁩ by MDN contributors.