Firefox 18 for developers

Firefox 18 was released on January 8, 2013. This article lists key changes that are useful not only for web developers, but also Firefox and Gecko developers as well as add-on developers.

Changes for Web developers

HTML

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

CSS

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). (bug 763689)
The cascade has been updated: now author !important rules override CSS animations. (bug 783714)
The background shorthand property now recognizes CSS3 background-size property specified inside. (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. (bug 666041)

DOM/APIs

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

JavaScript

Harmony's (ECMAScript 2015) Direct Proxies have been landed (bug 703537). Warning: the implementation contains a couple of known bugs, missing features and misalignements 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.

WebGL

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

SVG

MathML

XUL

Network

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

Developer tools

Changes for add-on and Mozilla developers

Interface changes

nsIStreamListener: The 4th parameter (aOffset) of onDataAvailable() method changes to unsigned long long. (bug 784912)
nsIUploadChannel: setUploadStream() supports over 2GB content-length (bug 790617)
nsIEditor: addEditorObserver() has been removed, use setEditorObserver() instead, removeEditorObserver() no longer takes a nsIEditorObserver parameter (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 (bug 800799)
nsIProtocolProxyService: The resolve method has been removed. Now, only the asyncResolve method can be used. See (bug 769764)

New interfaces

Removed interfaces

The following interfaces have been removed.

nsIEditorObserver