Firefox 27 for developers

Firefox 27 was released on February 4, 2014. 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.

Want to help document Firefox 27? See the list of bugs that need to be written about and pitch in!

Changes for Web developers

Developer Tools

  • Breakpoints can now be set on DOM events.
  • Javascript in the debugger panel can be unminified, using the { } button.
  • The inspector now has edit-element-html feature, without needing an addon.
  • Background-urls and colors have preview in inspector. Even hovering over canvas elements will give a popup with an image preview.
  • Reflow logging has been added.
  • Styles of SVG elements are now inspectable (bug 921191).
  • Failure to find the image when clicking url link in CSS inspector has been fixed (bug 921686).

More details in this post.


  • The -moz-grab and -moz-grabbing keywords on the CSS cursor property have been unprefixed to grab and grabbing (bug 880672).
  • Support for the -moz-hsla() and -moz-rgba() functional notations has been dropped. Only the unprefixed versions, hsla() and rgba() are supported from now on (bug 893319).
  • The "true" value for text-align has been added (bug 929991).
  • Experimental support of position:sticky is now active by default on non-release builds (bug 902992). For releases builds, the layout.css.sticky.enabled preference still needs to be set to true.
  • The all shorthand property has been added (bug 920633).
  • The unset global value has been added; it allows to reset any CSS property (bug 921731).
  • Curly brackets are no longer allowed in HTML style attributes: doing <div style="{ display: none }"> was working in quirk mode, but won't anymore bug 915053.
  • The overflow property now works on <fieldset> (bug 261037).


  • The color value of the <input> type attribute has been implemented on desktop platforms. It was already available on mobile ones.
  • The allow-popups directive is now supported with the sandbox attribute of the <iframe> element (bug 766282).
  • Blending of HTML elements using the mix-blend-mode property has been implemented. The layout.css.mix-blend-mode.enabled preference must be set to true (bug 902525).


EcmaScript 6 (Harmony) implementation continues!


  • Support for the two setRange() methods on the HTMLInputElement interface has been added (bug 850364).
  • Support for the two setRange() methods on the HTMLTextAreaElement interface has been added (bug 918940).
  • The method getAllKeys() and openKeyCursor() have been added to IDBObjectStore (bug 920633 and bug 920800).
  • The HTMLFormControlsCollection interface has been implemented (bug 913920).
  • The CanvasRenderingContext2D interface supports now the two methods getLineDash() and setLineDash() (bug 768067).
  • The typeMustMatch attribute has been implemented on the HTMLObjectElement interface (bug 827160).
  • The copyFromChannel() and copyToChannel() methods have been added to AudioBuffer (bug 915524).
  • Event.isTrusted() is now unforgeable (bug 637248).
  • The Navigator.vibrate() method has been adapted to match the final specification: It now returns false when the list is too long or has too large entries, instead of throwing (bug 884935).
  • As part of the ongoing effort to standardize global objects, the non-standard stylesheet change event interfaces, including StyleRuleChangeEvent, StyleSheetApplicableStateChangeEvent and StyleSheetChangeEvent, are no longer available from Web content. The CSSGroupRuleRuleList interface, the implementation detail of CSSRuleList, has also been removed (bug 872934 and bug 916871).
  • atob now ignores whitespaces (bug 711180).
  • WebGL: MOZ_ prefixed extension strings are deprecated. Support for them will be removed in the future. Use unprefixed extension string only. To get draft extensions, set the webgl.enable-draft-extensions preferences (bug 924176).


No change.


  • Blending of SVG elements using the mix-blend-mode property has been implemented. The layout.css.mix-blend-mode.enabled preference must be set to true (bug 902525).

Changes for addon and Mozilla developers

  • The downloads-indicator button has gone away. You should now use the downloads-button element. If you need to check that it has loaded its overlay, check for the indicator attribute on that button.
  • The chrome://browser/skin/downloads/indicator.css stylesheet is no longer referenced in Firefox.


  • TLS 1.2 has been implemented for improved security (bug 861266).

See also

Older versions

Document Tags and Contributors

Last updated by: teoli,