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Firefox 35 技術文件

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Firefox 35, based on Gecko 35, will ship in January 2015. This article provides information about the changes in this release that will affect developers. Nightly builds of what will become Firefox 35 are currently available on the Developer Edition channel.

Note: Items listed here are tentatively slated for Firefox 35; however, they may be held for a future release if testing shows they're not ready by the time Firefox 35 is due to ship. Please keep an eye on this page to stay up to date on the plans for Firefox 35.

Want to help document Firefox 35? See the list of bugs that need to be written about and pitch in!

Changes for Web developers

Developer Tools


All devtools bugs fixed between Firefox 34 and Firefox 35.



  • The obsolete and non-conforming bottommargin, leftmargin, rightmargin and topmargin attributes of the <body> element have been activated in non-quirks mode (bug 95530).


  • The "temporal dead zone" for let declarations has been implemented. In conformance with ES6 let semantics, the following situations
    now throw errors. See also this newsgroup announcement and bug 1001090.
    • Redeclaring existing variables or arguments using let within the same scope in function bodies is now a syntax error.
    • Using a variable declared using let in function bodies before the declaration is reached and evaluated is now a runtime error.
  • ES6 Symbols (only available in the Nightly channel) have been updated to conform with recent specification changes:
    • String(Symbol("1")) now no longer throws a TypeError; instead a string ("Symbol(1)") gets returned (bug 1058396).
  • The various TypedArray constructors now have as their [[Prototype]] a single function, denoted %TypedArray%  in ES6 (but otherwise not directly exposed).  Each typed array prototype now inherits from %TypedArray%.prototype.  (%TypedArray% and %TypedArray%.prototype inherit from Function.prototype and Object.prototype, respectively, so that typed array constructors and instances still have the properties found on those objects.)  Typed array function properties now reside on %TypedArray%.prototype and work on any typed array.  See TypedArray and bug 896116 for more information.
  • ES6 semantics for prototype mutations using object literals have been implemented (bug 1061853).
    • Now only a single member notated as __proto__:value will mutate the [[Prototype]] in the object literal syntax.
    • Method members like __proto__() {} will not overwrite the [[Prototype]] anymore.



  • The dtls OpenType feature (via the CSS font-feature-settings on the default stylesheet) is now applied automatically to MathML elements when positioning scripts over it (e.g. dotless i with mathematical hat).


No change.


No change.


Changes for add-on and Mozilla developers


  • The private _getTabForBrowser() method on the tabbrowser element has been deprecated. In its place, we've added a new, public, method called getTabForBrowser. This returns, predictably, the tab element that contains the specified browser.

Older versions


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