Firefox 39 for developers

Firefox 39 was released on July 2, 2015. 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

Developer Tools

Highlights:

• WebIDE now supports debugging Firefox OS devices over Wi-Fi
• WebIDE now supports Cordova projects
• Animations view: rewind, fast-forward, and jump to a specific time
• The cubic Bézier curve editor now includes 31 presets
• Drag and drop elements in the Page Inspector
• Web console command history is now persisted across sessions
• $_ console command to print the last result evaluated
• Better box model highlighting for inline elements

All devtools bugs fixed between Firefox 38 and Firefox 39.

CSS

• Support of the <string> type on list-style-type, as well as its shorthand property list-style has been implemented (bug 1144607).
• CSS Scroll Snapping has been implemented (bug 945584 and bug 1138658).
• Cascading of CSS Animations and CSS Transitions has been rewritten to match the latest specification (bug 1125455).
• Support for vertical scripts, with the writing-mode is now turned on by default on Nightly and Developer Edition, but not on Firefox Beta and Firefox Release (bug 1099032). Note that implementation is not yet complete and some widgets, like tables, will not obey to all values.
• CSS filter acts like position: relative + overflow: hidden (bug 1125767).

HTML

• A new ARIA role, switch, is now supported (bug 1136563).
• Support for <link rel="preconnect"> allowing to anticipate a future connection without revealing any information has been implemented (bug 1135160).

JavaScript

• The RegExp constructor no longer throws when the first argument is a RegExp and the second flags argument is present (bug 1108949).
• The Object.prototype.__noSuchMethod__ property is now deprecated and throws a console warning (see bug 1140428 and this platform announcement).
• The implementation of the Proxy object has been updated to comply more with the ES2015 specification:
  • The defineProperty and set handlers now need to explicitly return true to be successful, otherwise a TypeError exception will be thrown in strict mode (bug 1132522).
  • If the window object is set as the target, those handlers will now throw a TypeError (bug 828137).
• When using arrow functions (=>), a line terminator (\n) is no longer allowed after arrow function arguments (() \n => {}) (bug 1141392).
• RegExp.prototype.toString is now a generic function (bug 1079919).
• The non-standard flags argument of String.prototype.match(), String.prototype.search(), and String.prototype.replace() is now deprecated and throws a console warning (bug 1142351).
• The try/catch behavior of Object.assign() has been removed to conform with the latest ES2015 draft (bug 1103344).

Interfaces/APIs/DOM

• The experimental CanvasRenderingContext2D.addHitRegion() method now accepts a path option, which allows you to add hit regions to Path2D objects (bug 1129147).
• New methods have been added to manipulate FormData objects (bug 1085283) and FormData is now supported in Web workers (bug 739173).
• The non-standard XMLHttpRequest.sendAsBinary() method has been removed (bug 853162).
• Progress in our experimental implementation of Web Animations: AnimationPlayer.startTime is now writeable (bug 1073379).
• Progress in our experimental implementation of Service Workers: Cache and CacheStorage interfaces are now implemented (bug 940273).
• The experimental Fetch API has been activated by default (bug 1133861).
• Progress in our experimental implementation of WebGL2: WebGLSync is now implemented (bug 1048721).
• Support for MouseEvent.offsetX and MouseEvent.offsetY have been added on desktop (bug 69787, but not on Firefox for Android or Firefox OS (they will be added in Firefox 43).

MathML

No change.

SVG

No change.

Audio/Video

No change.

Miscelleanous

• WOFF2 fonts support is now turned on by default in release version of Firefox (Beta and Release, in addition of Nightly and Developer Edition) (bug 1084026).
• The -remote command-line option has been removed (bug 1080319).

• Support for new Unicode 8.0 skin tone emoji (bug 1153460).

Networking

• Support for SSLv3 has been completely removed (bug 1106470).

Security

No change.

Changes for add-on and Mozilla developers

No change.

See also

• Site Compatibility for Firefox 39

Older versions