Introducing Firefox Development Highlights

We know he have a lot of readers out there interested in the Open Web and its capabilities, and part of that is to see the latest additions and implemented features in Firefox. Therefore, we’re introducing Firefox Development Highlights here at Mozilla Hacks.


The purpose of this post is to highlight some of the latest (couple of weeks) developments in Firefox and Gecko. Bear in mind that these changes might be backed out at any time and they are part of our Bleeding Edge posts.

You can test them out in Firefox Nightly.

If you are interested in a steady flow of the latest highlights, you can also follow @FirefoxNightly on Twitter.


Firefox’ graphic stack received some great improvements lately: better scaling algorithm for images and support for retina display. Gecko now downscales images using a high-quality scaling algorithm. You can see the improvements in this screenshot (click on it for a larger version):

This is disabled on mobile, where we worry about speed/multicore abilities. It’s also disabled on OS X, which has high-quality downscaling built in.

Gecko now supports HiDPI displays, for web pages, plugins and for Firefox’ UI.


CSS3 flexbox model is now available in Gecko, behind a preference: layout.css.flexbox.enable. Go to about:config in Firefox and add that as a preference, set to true.

Quote from W3C:

In the flex layout model, the children of a flex container can be laid out in any direction, and can “flex” their sizes, either growing to fill unused space or shrinking to avoid overflowing the parent. Both horizontal and vertical alignment of the children can be easily manipulated. Nesting of these boxes (horizontal inside vertical, or vertical inside horizontal) can be used to build layouts in two dimensions.

Web Workers

Support for transferable objects from HTML5 spec.

Quote from Andrea Marchesini:

If you want to send data to/from a Web Worker, you have to use postMessage() method. Internally what happens is that the data is duplicated using the structured cloned algorithm and then the copy is sent. To make this sharing faster, HTML5 specs add a new concept: transferable objects, data is transferred from one context to another without copy. Note: data is no longer available once transferred to the new context. Right now we can transfer just ArrayBuffers, but maybe in the future we will support other data types.

About Paul Rouget

Paul is a Firefox developer.

More articles by Paul Rouget…

About Robert Nyman [Editor emeritus]

Technical Evangelist & Editor of Mozilla Hacks. Gives talks & blogs about HTML5, JavaScript & the Open Web. Robert is a strong believer in HTML5 and the Open Web and has been working since 1999 with Front End development for the web - in Sweden and in New York City. He regularly also blogs at and loves to travel and meet people.

More articles by Robert Nyman [Editor emeritus]…


  1. kris

    why Firefox not support dark theme as IE. I thing this is GECKO issue but other software at-least perform good at dark theme.

    any future plan to make it like how chrome have done it.

    October 10th, 2012 at 10:44

    1. Robert Nyman

      A dark theme for which part? The web browser in general?
      The developer tools have a dark theme in the latest Nightly.

      For the web browser in general, I recommend checking out Personas for Firefox.

      October 10th, 2012 at 12:46

  2. kris

    oops, I make some confusion..

    Suppose in Win7 > personalize > themes > High contrast.

    After doing it I am only have seen black and yellow color in Firefox.

    for screenshot take a look

    October 10th, 2012 at 20:59

    1. Robert Nyman

      Ah, I see. Best way forward is to talk to the Firefox support.

      October 11th, 2012 at 03:26

  3. Ken Saunders

    Firefox has had native support for O.S. high contrast themes for a very long time now. It was an accessibility issue.
    I just checked all of the Windows 7 High Contrast themes and Firefox follows along.
    “After doing it I am only have seen black and yellow color in Firefox.”
    That is the Windows theme.
    You can customize all of the colors by selecting “Window Color” in the bottom row of the Windows Personalization windows.
    You can also customize the global colors in Firefox by going to Options > Content > Fonts & Colors”.

    October 11th, 2012 at 18:29

    1. Robert Nyman

      Thanks for testing, Ken!

      October 11th, 2012 at 23:41

  4. Jay

    I think the BEST dark theme (and much much more) is Foxdie “IronMan”

    It says it don’t work on nightly, but you know the drill, Tools | nightly tester tools | disable | Force addon Compatibility..

    Judge it….

    January 24th, 2013 at 16:21

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