Using the B2G desktop client

The Firefox OS desktop client, also called the B2G desktop client, lets you run Gaia and web apps in a Gecko-based environment somewhat similar to an actual device. It doesn't emulate device hardware, so it's not adequate for testing device APIs, and isn't a replacement for testing on actual hardware. However, it does have a several APIs enabled that aren't available on Firefox such as the Contacts and Settings APIs. It can therefore be useful during the development of our application, or while working on the Gaia user interface itself.

This article covers downloading or building the Firefox OS desktop client, as well as how to use it.

Download a nightly build

Just like Firefox Nightlies, the Firefox OS desktop client is built every day from the latest source code. The latest build is available from the Mozilla FTP server. Be sure to pick the latest version and the right archive for your operating system.

We can now skip ahead to Running the desktop client.

Building the desktop client

The first thing we need to do is set up a standard Mozilla build environment. Once we have that, we can pull down the code we'll need and configure to build the Firefox OS desktop client.

Downloading the code for the first time

In a directory where we'd like the source code to go, let's clone the mozilla-central repository that contains all of Gecko:

 hg clone

Updating the code

When we do subsequent builds later, we'll want to make sure we have the latest code. Here's how to pull the latest changes:

cd mozilla-central
hg pull -u

Create a mozconfig

Next, we need to create a mozconfig file in the mozilla-central directory to configure the build system to build the Boot to Gecko client instead of Firefox:

mk_add_options MOZ_OBJDIR=../build
mk_add_options MOZ_MAKE_FLAGS="-j9 -s"

ac_add_options --enable-application=b2g
ac_add_options --disable-libjpeg-turbo
ac_add_options --enable-tests

# turn on mozTelephony/mozSms interfaces
# make sure this line is commented.
#ac_add_options --enable-b2g-ril


Now we're ready to build the desktop client with the following command issued from the mozilla-central directory:

make -f

The built client will be placed in the ../build/dist directory (based on the value you specify for MOZ_OBJDIR in the mozconfig file).

Running the desktop client

By default the desktop client will show an empty screen because it doesn't know which web app to load initially as the system app. The collection of system apps and default apps that come with Firefox OS is called Gaia.

Downloading Gaia

To download Gaia for the first time, let's clone the source code repository on GitHub:

 git clone

To update an already existing clone of Gaia, we can pull in the latest changes from GitHub:

cd gaia
git pull

Generating a profile

Next we need to set up Gaia's apps for the desktop client. This includes packaging the Gaia apps in the same way like they would be installed on the device, as well as setting up the permissions for the privileged system apps. We do this by generating a profile. The following command will take care of that: The new profile contains a customized extension and other configuration needed to make B2G run properly. So do this in the gaia directory:

cd gaia

This should create a profile directory below the gaia directory.

Running on Linux

To run the desktop client against the Gaia profile we just generate, we simply invoke the b2g binary with the profile parameter. The binary will be located in the tarball we downloaded earlier or in the ../build/dist/bin directory if we built the client ourselves.

.../b2g -profile gaia/profile


user_pref("layers.acceleration.disabled", true);

Running on Mac

On Mac, the command line is slightly more complicated due to the location of the b2g binary and the need for absolute paths when specifying the profile directory:

.../ -profile /full/path/to/gaia/profile

Command line options

There are a number of command line options you can use to adjust the runtime experience while using the desktop client. You can get a list by using the -help option. This section covers some of the particularly interesting ones.

Specifying the screen size

You can specify the screen size of the device you want to simulate using the --screen option:

b2g --screen=<width>x<height>[@<dpi>]

Where <width>, <height>, and <dpi> are fairly self-explanatory parameters: the width and height of the device's screen in pixels and the device resolution in DPI. For example:

b2g --screen=320x480
b2g --screen=320x480@160

Optionally, you can specify certain devices by name to simulate their screen size and resolution:

  • iphone
  • ipad
  • nexus_s
  • galaxy_nexus
  • galaxy_tab
  • wildfire
  • tattoo
  • salsa
  • chacha

Opening the JavaScript console

You can open the JavaScript console when launching the desktop B2G client by launching it from the command line with the -jsconsole flag. After building, just do:

.../b2g -jsconsole -profile /path/to/your/profile

If you've installed the nightly build on a Mac, you can do the following:

/Applications/ -jsconsole -profile /path/to/your/profile

Launching a specific application at startup

You can now specify an application to be launched automatically when b2g starts up in the desktop client. This is done as soon as the rest of the system is done loading up. To do this, just use the --runapp option, which takes as a parameter the name of the application to run. For example:

 .../b2g -profile /path/to/your/gaia/profile --runapp email

Before looking for an app to launch, the specified name is normalized by converting it to all lower case and removing all dashes and spaces. This normalized name is then compared to similarly normalized names from the manifests of available apps' manifests.

For example, the name of the email app is currently "E-mail", but --runapp email will work because of this normalization.

If you specify the --runapp option without an argument, or with an empty argument, the b2g client will output to your terminal a list of the known applications as well as a brief usage message.

Note: Using the --runapp option disables the lock screen as a side effect and does not re-enable it. It's assumed that you won't use this command on a profile on which you will be testing the lock screen, or you will turn it back on manually. Feel free to contribute a patch to change this behavior if it's a problem.

Usage tips


  • The Home key performs the same function as the "home" button; if you're on a Mac, the Home key is available as Fn+← (Fn + Left Arrow).

Next steps


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