Developer Edition 42: Wifi Debugging, Win10, Multiprocess Firefox, ReactJS tools, and more

Firefox 42 has arrived! In this release, we put a lot of effort into the quality and polish of the Developer Edition browser. Although many of the bugs resolved this release don’t feature in the Release Notes, these small fixes make the tools faster and more stable. But there’s still a lot to report, including a major change to how Firefox works.

Debugging over wifi

Now, with remote website debugging, you can debug Firefox for Android devices over wifi – no USB cable or ADB needed.

Multiprocess is enabled by default

Multiprocess Firefox (aka E10s) has been enabled by default in Developer Edition. When it’s enabled, Firefox renders and executes web-related content in a single background content process. If you experience any issues with addons after updating to Developer Edition 42, try disabling incompatible addons or reverting to a single process mode using about:preferences.

Windows 10 theme support

The Developer Edition theme has a new look in Windows 10 to match the OS styling. Take a look:

Screenshot of the dark Developer Edition theme in Windows 10

Dark Developer Edition theme – Windows 10

Screenshot of the light Developer Edition theme in Windows 10

Light Developer Edition theme – Windows 10

React Developer Tools support for Firefox

If you’re developing with ReactJS, you may have noticed that the React project recently released a beta for their developer tools extension, including initial support for Firefox. While there are no official builds yet of the Firefox version, the source is available on github.

Other notable changes

  • Asynchronous call stacks now allow you to follow the code flow through setTimeout, DOM event handlers, and Promise handlers. (Bug 981514)
  • There is a new configurable Firefox OS simulator page in WebIDE. From here, you can change a simulator to run with a custom profile and screen size, using a list of presets from reference devices. (Bug 1156834)
  • CSS filter presets are now available in the inspector. (Bug 1153184)
  • The MDN tooltip now uses syntax highlighting for code samples. (Bug 1154469)
  • When using the “copy” keyboard shortcut in the inspector, the outerHTML of the selected node is now copied onto the clipboard. (Bug 968241)
  • New UX improvements have landed in the style editor’s search feature. (Bug 1159001, Bug 1153474)
  • CSS variables are now treated as normal declarations in the inspector. (Bug 1142206)
  • CSS autocomplete popup now supports pressing ‘down’ to list all results in an empty value field (Bug 1142206)

Thanks to everyone who contributed time and energy to help the DevTools team in this release of Firefox Developer Edition 42! Each release takes a lot of effort from people writing patches, testing, documenting, reporting bugs, sending feedback, discussing features, etc. You can help set our priorities by sharing constructive feedback and letting us know what you’d like from Firefox Developer Tools.

You can download Firefox Developer Edition now, for free.

About Brian Grinstead

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  1. Ult Combo

    The Windows 10 Developer Edition theme is too ugly, IMHO. Can we opt-out of it or opt-in to the old dark theme?

    August 24th, 2015 at 12:38

    1. Dan Callahan

      To switch between light and dark themes, open up the Developer Tools (F12), click the settings button (gear icon) on the right, and look under the “Theme Settings” header.

      Out of curiosity, what changes do you dislike? There should have only been minor changes for Windows 10…

      August 24th, 2015 at 13:00

      1. Ult Combo


        My bad, I rushed my previous comment a bit too much. The Windows 10 theme looks awesome! (both light and dark versions)

        Thing is, 2 weeks or so ago I was using the Developer Theme on my Nightly, and then when I upgraded to Windows 10, it immediately switched to a ugly’ish light theme. Now, trying out the theme with the Windows 10 tweaks, everything looks perfectly on both light & dark themes. :D

        Congratz everyone!

        August 24th, 2015 at 15:41

        1. Tim Nguyen

          If you have installed, that ugly’ish light theme is shown because Firebug doesn’t properly support the Developer Theme.

          August 24th, 2015 at 16:20

          1. Ult Combo

            Yes, that was it. I believe the Developer theme was failing to load and thus Nightly was falling back to the default theme (in this case, the Developer theme’s light/dark switch has no effect).

            The Developer theme seems to be working fine in Nightly for the moment, though. ;)

            August 24th, 2015 at 16:24

  2. Lucas


    August 27th, 2015 at 12:21

  3. Alexandre

    What happened to the browser debugger? (⌘-⌥-⇧-i)

    August 28th, 2015 at 00:24

    1. Brian Grinstead

      It’s still there, but you may need to enable browser chrome and remote debugging to get it to work –

      August 28th, 2015 at 08:30

  4. Nickolay

    > Asynchronous call stacks now allow you to follow the code flow through setTimeout, DOM event handlers, and Promise handlers. (Bug 981514)

    How/where can these be tested? I don’t see these in the nightly (debugger, nor console.trace()) with javascript.options.asyncstack=true. I run nested setTimeout’s to test.

    August 29th, 2015 at 14:45

    1. Brian Grinstead

      That’s a great question. I’ve just confirmed locally that I’m only seeing ‘bar’ in the stack trace for console / debugger when calling:

      setTimeout(function foo() {
      setTimeout(function bar() {
      }, 100);
      }, 100);

      I’m going to ask around to double check if there is something else we need to do to enable this.

      August 31st, 2015 at 09:16

    2. Brian Grinstead

      I’ve confirmed that this isn’t working as expected and filed a bug for further investigation: Thanks for reporting!

      September 1st, 2015 at 14:20

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