To test the latest developer features of Firefox, install Firefox Developer Edition Firefox 40 was released on August 11, 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:

• Improvements to the Animations view
• Get help from MDN for CSS property syntax
• Edit filters in the Page Inspector
• Web Console now shows messages from workers
• Filter requests by URL in the Network Monitor
• Many new context menu options in the Network Monitor
• Show when network resources are fetched from the browser cache
• Filter rules in the Page Inspector

More:

• Break at debugger; statements in unnamed eval sources
• Copy URL/Open in New Tab context menu items for Debugger source list pane
• console.dirxml support in the Web Console
• Style Editor: "Open Link In New Tab" item added to stylesheet list
• Inspector selector search now includes class/id results even without css prefix
• Tooltips in box-model view saying which CSS rule caused the value
• Switch between color unit format in the Inspector using Shift+click
• Implement "Scroll Into View" menu item for the Inspector
• Linkify url/id/resource attributes in the Inspector
• IP address tooltip in the Network Monitor

Everything:all devtools bugs fixed between Firefox 39 and Firefox 40.

CSS

• Prefixed rules (-moz-) fortext-decoration-color,text-decoration-line, andtext-decoration-style have been removed (Firefox bug 1097922).
• The propertytext-align now supports thematch-parent value (Firefox bug 645642).
• In Quirks Mode,empty-cells now defaults toshow, like in standard mode (Firefox bug 1020400).
• The-moz-orient non-standard property, used to style<meter> and<progress> element has been adapted for vertical writing-modes: the valueauto has been dropped and the valuesinline andblock added, withinline being the new default value (Firefox bug 1028716).
• The propertyfont-size-adjust has been fixed so that0 is treated as a multiplier (leading to a0 height for the font, hence hiding it) instead of thenone value (leading to no adjustment, or a1.0 value) (Firefox bug 1144885).
• Fix text-overflow doesn't work in vertical writing mode (Firefox bug 1117227).

HTML

No change.

JavaScript

• Unreachable code afterreturn statement (including unreachable expression aftersemicolon-less return statements) will now show a warning in the console (Firefox bug 1005110,Firefox bug 1151931).
• Symbol.match has been added (Firefox bug 1054755).
• Passing an object which has a property namedSymbol.match with atruthy value toString.prototype.startsWith,String.prototype.endsWith, andString.prototype.contains now throws aTypeError (Firefox bug 1054755).
• RegExp function returns pattern itself if called withoutnew and pattern object has a property namedSymbol.match with atruthy value, and the pattern object'sconstructor property equals toRegExp function. (Firefox bug 1147817).
• Support for the non-standard JS1.7 destructuring for-in has been dropped (Firefox bug 1083498).
• Non-standard initializer expressions infor...in loops are now ignored and will present a warning in the console. (Firefox bug 748550 andFirefox bug 1164741).
• \u{xxxxxx} Unicode code point escapes have been added (Firefox bug 320500).
• String.prototype.contains has been replaced withString.prototype.includes,String.prototype.contains is kept as an alias (Firefox bug 1102219).
• If theDataView constructor is called as a function without thenew operator, aTypeError is now thrown as per the ES2015 specification.
• An issue regressed in Firefox 21, where proxified arrays without theget trap were not working properly, has been fixed. If theget trap in aProxy was not defined,Array.length returned0 and theset trap didn't get called. A workaround was to add theget trap even if was not necessary in your code. This issue has been fixed now (Firefox bug 895223).
• WeakMap.prototype andWeakSet.prototype have been updated to be just ordinary objects, per ES2015 specification (Firefox bug 1055473).

Interfaces/APIs/DOM

New APIs

• ThePush API has been experimentally implemented (Firefox bug 1038811). Controlled by theservices.push.enabled pref, it is disabled by default.

Web Animations API

Improvement in our experimental Web Animations implementation, mostly to match latest spec changes:

• AnimationPlayer.currentTime now can also be set (Firefox bug 1072037).
• Animatable.getAnimationPlayers(), available onElement has been renamed toElement.getAnimations() (Firefox bug 1145246).
• Animation andAnimationEffect have been merged into the newly createdKeyframeEffectReadOnly (Firefox bug 1153734).
• AnimationPlayer has been renamed toAnimation (Firefox bug 1154615).
• AnimationTimeline is now an abstract class, withDocumentTimeline its only implementation (Firefox bug 1152171).

CSSOM

• The CSS Font Loading API is now enabled by default in Nightly and Developer Edition releases (Firefox bug 1088437). It is still deactivated by default in Beta and Release browsers.
• TheCSSCharsetRule interface has been removed and such objects are no longer available in CSSOM (Firefox bug 1148694). This matches the spec (recently adapted) and Chrome behavior.

WebRTC

• WebRTC: thenegotiationneeded event is now also sent for initial negotiations, not only for re-negotiations (Firefox bug 1149838).

DOM & HTML DOM

• When unable to parse thesrcset, theHTMLImageElement.currentSrc method doesn't returnnull anymore but"", as requested by the latest specification (Firefox bug 1139560).
• Like for images, Firefox now throttlesWindow.requestAnimationFrame() for non-visible<iframe> (Firefox bug 1145439).
• Navigator.taintEnabled is no longer available for Web workers (Firefox bug 1154878).

Web Audio API

New extensions to theWeb Audio API:

• TheAudioContext.state andAudioContext.onstatechange properties as well as the methodsAudioContext.suspend(),AudioContext.resume(), andAudioContext.close() have been added (Firefox bug 1094764).
• AudioBufferSourceNode now implements theAudioBufferSourceNode.detunek-rate attribute (Firefox bug 1153783).

Web Workers

• Slight improvement in ourService Worker API: theupdate() method has been moved fromServiceWorkerGlobalScope toServiceWorkerRegistration (Firefox bug 1131350).
• ServiceWorkerRegistration is now available in Web workers (Firefox bug 1131327).
• DataStore is now available in Web workers (Firefox bug 916196).

IndexedDB

• IDBTransaction are now non-durable by default (Firefox bug 1112702). This favors performance over reliability and matches what other browsers are doing. For more information, read ourdurability definition.

Dev Tools

• The propertyconsole.timeStamp() has been added (Firefox bug 922221).

MathML

No change.

SVG

No change.

Audio/Video

No change.

Networking

No change.

Security

• Using an asterisk (*) in aCSP does not include the schemesdata:,blob: or:filesystem anymore when matching source expressions. So those schemes now need to be explicitly defined within the related header to match the CSP (Firefox bug 1086999).

Changes for add-on and Mozilla developers

XUL

• It is no longer possible to create transparent top-level windowsFirefox bug 1162649.

JavaScript code modules

• Dict.jsm has been removedFirefox bug 1123309. UseMap instead.

XPCOM

• ThensIClassInfo.implementationLanguage attribute has been removed, along with thensClassInfo::GetImplementationLanguage() function.
• The following XPCOM interfaces have been removed; you should use the standard HTML interfaces instead:
  • nsIDOMHTMLBRElement
  • nsIDOMDivElement
  • nsIDOMHTMLHeadingElement
  • nsIDOMHTMLTableCaptionElement
  • nsIDOMHTMLTableElement
  • nsIDOMHTMLTitleElement

Other

• Places Keywords API has been deprecated and will be removed soon (Firefox bug 1140395).
• The automated testing system now supports skipping individual test functions. Seerunning conditional tests in XPCShell testing.