Firefox 27 for developers

• 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 (Webkit bug 921191).
• Failure to find the image when clicking URL link in CSS inspector has been fixed (Webkit 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 brackets 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).

• 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 (Firefox 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 (Firefox bug 902525).
• The typeMustMatch property of the <object> element is now supported (Firefox bug 827160).

ECMAScript 2015 implementation continues!

• The spread operator is now supported in Function calls (Firefox bug 762363).
• The mathematical function Math.hypot() has been implemented (Firefox bug 896264).
• The yield* expression is now implemented (Firefox bug 666396).
• The MapIterator, SetIterator and ArrayIterator objects now match the specification (Firefox bug 881226).
• for...of loops now expect the ES2015 standard iterator protocol moving away from SpiderMonkey old iterator protocol using StopIteration.
• String.match and String.replace now reset RegExp.lastIndex (Firefox bug 501739).

• Support for the two setRange() methods on the HTMLInputElement interface has been added (Firefox bug 850364).
• Support for the two setRange() methods on the HTMLTextAreaElement interface has been added (Firefox bug 918940).
• The methods getAllKeys() and openKeyCursor() have been added to IDBObjectStore (Firefox bug 920633 and Firefox bug 920800).
• The HTMLFormControlsCollection interface has been implemented (Firefox bug 913920).
• The CanvasRenderingContext2D interface now supports the two methods getLineDash() and setLineDash() and the lineDashOffset property (Firefox bug 768067).
• The typeMustMatch attribute has been implemented on the HTMLObjectElement interface (Firefox bug 827160).
• The copyFromChannel() and copyToChannel() methods have been added to AudioBuffer (Firefox bug 915524).
• Event.isTrusted() is now unforgeable (Firefox bug 637248).
• The WebRTC API's RTCIceCandidate object now includes a toJSON() method to help with signaling and debugging (Firefox bug 928304).
• 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 (Firefox 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 (Webkit bug 872934 and Firefox bug 916871).
• atob now ignores whitespaces (Firefox 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 (Firefox 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 (Firefox bug 902525).

• 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 (Webkit bug 861266).

• List of changes in Marionette for Firefox 27.