Introducing DevTools Reload

As a way to encourage more open source developers to contribute to the evolution of Firefox Developer Tools, the DevTools team has built DevTools Reload, an add-on that removes some common barriers to entry.

It’s not unusual for mature Open Source projects to have large code bases. This can act as a barrier to entry for new contributors. It might feel as though so much code is going to take a long time to learn, so even if the best way to get started is by fixing simple bugs, the task can seem overwhelming due to the size of the code base.

Another challenge with large code bases is that they often take a great deal of time to download and compile, which further discourages new contributors from taking part. Even if someone decides to dive in at this point, long recompile times after making simple changes can discourage even the most patient of developers.

DevTools Reload allows developers to make changes to the JavaScript and CSS source files, and by pressing a short-cut key combination, automatically reload the tools with all the new changes. Firefox thinks the tools should be hackable just like the Web.

This approach is convenient for making simple fixes to Firefox dev tools, and provides almost instant feedback to the developer who is making the change, allowing for quick iteration and experimentation on possible solutions.

Here’s a short screencast showing how to install the add-on on Firefox Nightly and demonstrating the feature. Or, if you prefer the written word, we also have some guidance on how to get started.

This feature is fairly new, and is still under development, so you might find rough edges and inconsistencies. Please file bugs if you find something doesn’t quite work as you expect it, or get in touch. You can follow the DevTools team (@firefoxdevtools) for more updates. Make sure to submit your contributions to the tools through bugzilla.

About Soledad Penadés

Sole works at the Developer Tools team at Mozilla, helping people make amazing things on the Web, preferably real time. Find her on #devtools at

More articles by Soledad Penadés…


  1. Colby Russell

    When the devtools team first decided to go off into their own corner and start working on a new inspector, I tried getting them to keep it separable instead of baking it into Firefox for exactly this reason, and others. It’s good that this is happening now, even if it’s late.

    Now we just need to do the same for the Firefox UI, so that contributors don’t have to build all of Gecko before starting to make changes.

    February 24th, 2016 at 16:46

  2. Šime Vidas

    Can DevTools itself be inspected by another instance of DevTools (like in Chrome)?

    February 24th, 2016 at 20:42

    1. Soledad Penadés

      @Sime yes! it’s documented here
      I’d like to make a video showing this sometime soon. It’s pretty rad and totally indispensable when working on the tools.

      February 25th, 2016 at 02:56

  3. fvsch

    @Šime Vidas: yep. In the devtools options, you need to enable two advanced settings:

    1. “Enable remote debugging”
    2. “Enable browser chrome and add-on debugging toolboxes”

    Then you can launch Tools > Web Developer > Browser Toolbox.

    February 25th, 2016 at 00:48

  4. Katie Lyons

    Thank you DevTool Team that provide this tool that make us easy to develop website and make Firefox more developer friendly.

    From 2006 until now, Firefox still is my favorite Developer Browser.

    February 25th, 2016 at 18:12

  5. Dane MacMillan

    I might be missing a critical step in this new process, but at what point–and how–do the changes being made make their way back upstream?

    Is this new process *only* for providing quick iteration/experimentation? Any promising results this testing produced must then be copied and pasted into the actual mercurial repository, cloned separately and located elsewhere on disk, then pushed up, is that right?

    The largest barrier to entry is the use of mercurial, and not because it’s a difficult tool, but because it doesn’t offer the familiarity that GitHub does. Today, any newcomer who fancies themselves a developer or developer-to-be will almost certainly use GitHub as a springboard for this pursuit.

    February 26th, 2016 at 07:14

    1. Soledad Penadés

      This feature only covers reloading the tools on the fly. Writing a patch and contributing upstream is something we want to cover in future posts/screencasts.

      But if you want to have a go today, you don’t need to use Mercurial! We maintain a git mirror (which is showed in the screencast) and you can send initial patches against that one following the process here:

      There’s also work on removing the Mercurial requirement and simplifying the patch submission process too – we are aware of its awkwardness! :-)

      February 26th, 2016 at 09:30

  6. lushijie

    which key to save the change( you note by pressing a short-cut key combination )? When I change the css,it does not work untill I restall the rdf.

    March 11th, 2016 at 22:25

  7. green software tech

    really good workplace for coders i use it and this platform give me a very good coding experience.

    March 20th, 2016 at 05:12

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