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.

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 an "edit-element-html" feature, without needing an add-on.
  • Background-URLs and colors have preview in inspector. Even hovering over canvas elements will give a pop-up with an image preview.
  • Reflow logging has been added.
  • Styles of SVG elements are now inspectable (Firefox bug 921191).
  • Failure to find the image when clicking URL link in CSS inspector has been fixed (Firefox bug 921686).
  • The X-SourceMap header is now supported (Firefox bug 765993).

More details in this post.


  • The -moz-grab and -moz-grabbing keywords on the CSS cursor property have been unprefixed to grab and grabbing (Firefox 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 (Firefox bug 893319).
  • The "true" value for text-align has been added (Firefox bug 929991).
  • Experimental support of position:sticky is now active by default on non-release builds (Firefox 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 (Firefox bug 842329).
  • The unset global value has been added; it allows to reset any CSS property (Firefox bug 921731).
  • Curly braces are no longer allowed in HTML style attributes: doing <div style="{ display: none }"> was working in quirks mode, but won't anymore Firefox bug 915053.
  • The overflow property now works on <fieldset> (Firefox bug 261037).



ECMAScript 2015 implementation continues!



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 (Firefox 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.


See also

Older versions