A Mozilla App Outside Central

After forking gecko-dev again for an experiment, I wondered if there was a better way to create a Mozilla app outside the mozilla-central repository. After all, that repository, mirrored as mozilla/gecko-dev on GitHub, is huge. And forking the same repo twice to the same GitHub account (or organization) requires cumbersome workarounds (and workarounds for the workarounds in some cases).

Plus, Mozilla apps don’t necessarily need to modify Gecko. But even when they do, they could share a single fork, with branches for each app, if there was a way to create a Mozilla app that imported gecko-dev as a dependency without having to live inside a fork of gecko-dev itself.

So I created an example app that does just that. It happens to use a shallow Git submodule to import gecko-dev, but it could use a Git subtree or any other dependency management tool.

The build process touches only one file in the gecko-dev subdirectory: it creates a symlink to the parent (app) directory to work around the limitation that Mozilla app directories must be subdirectories of gecko-dev.

To try it for yourself, clone the mykmelez/mozilla-app repo and build it:

git clone --recursive --shallow-submodules https://github.com/mykmelez/mozilla-app
cd mozilla-app/
./mach build && ./mach run

You should see a window like this:

Mozilla app window

And your clone will contain the app’s files, the gecko-dev submodule directory, and an obj directory containing the built app:

> ls
LICENSE  app.mozbuild  components   gecko-dev  modules    moz.configure  obj
app      branding      confvars.sh  mach       moz.build  mozconfig


The app builds Gecko from source, so you still need Mozilla build prerequisites. On some platforms, you can install these via ./mach bootstrap.

I’ve only tested it on Mac, but it probably works on Linux, and it probably doesn’t work on Windows (because of the symlink).

Although the Mozilla application framework is mature and robust, it isn’t explicitly generalized for use beyond Firefox anymore, and it could change or break at any time.


Myk Melez

Myk is a Principal Software Architect and in-house entrepreneur at Mozilla. A Mozillian since 1999, he's contributed to the Web App Developer Initiative, PluotSorbet, Open Web Apps, Firefox OS Simulator, Jetpack, Raindrop, Snowl, Personas, Firefox, Thunderbird, and Bugzilla. He's just a cook. He's all out of bubblegum.


2 thoughts on “A Mozilla App Outside Central

  1. Impressive, Myk. Since sub-repositories are not well-liked in Mercurial (officially a “feature of last resort”), this may be an interesting advantage of git over hg.

    (I still prefer hg, though.)

    Your observation about Mozilla no longer being generalized for use beyond Firefox matches one of those bits of unease I have about becoming obsolete: I built a good career on XUL and XPCOM, and I’m getting the sense they’re no longer loved. I’m of course happy that they dumped CVS as a version control system… but on practically every other aspect of working with Mozilla code, “best practices” are changing faster than I can keep up. Which is scary, because it’s always been my job to keep whatever codebase I work with compatible with Mozilla…

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