This page describes the Firefox OS Simulator for developers targeting Firefox OS 1.2 or later. If you're developing apps for Firefox OS 1.1, then you should check out the documentation for the Firefox OS 1.1 Simulator instead.

The Firefox OS Simulator is a version of the higher layers of Firefox OS that simulates a Firefox OS device, but runs on the desktop. This means that in many cases, you don't need a real device to test and debug your app. It runs in a window the same size as a Firefox OS device, includes the Firefox OS user interface and built-in apps, and simulates many of the Firefox OS device APIs.

The Simulator is packaged and distributed as a Firefox add-on. Once you've downloaded it and installed it in Firefox, you can run it, push apps to it, and attach the developer tools to it using the App Manager tool or WebIDE.


To install the simulator, use WebIDE's Manage Extra Components pane (available as part of Firefox 34 and onwards). Multiple versions are available, and you are advised to install them all, for maximum flexibility.

Starting with Firefox 43, the about:addons page may display warnings about your addon being unsigned. The simulators are properly signed at install time, however they are subsequently modified and thus appear "broken". They will continue to run successfully despite this warning message. Details are in bug 1197262.

To start the Simulator, you choose it from WebIDE's runtime list. For more details, see the instructions in the WebIDE documentation. Once the Simulator's running, you can push apps to it and debug them using the WebIDE, just as you can with a real device.

If you are using the App Manager (an older tool available prior to the WebIDE), you can install a simulator via the following button:

Install Simulator

The Simulator UI

The Simulator appears as a separate window, sized so the simulated screen area is 320x480 pixels. To simulate touch events you can click the mouse button and drag while holding the button down. So by clicking and dragging right-to-left from the Home Screen, you'll see the built-in apps, as well as any apps you have added:

The Simulator has two buttons in a toolbar at the bottom:

  • the button on the left takes you to the Home screen, or switches the Simulator off if you hold it down
  • the button on the right switches the Simulator between portrait and landscape orientation. This will generate the orientationchange event.

SD card emulation

In the Simulator the device SD card is mapped to the "fake-sdcard" directory in the Simulator's profile, which is itself located inside the "extensions" directory under the Firefox profile in which the Simulator is installed. For example:


Files read or written using the getDeviceStorage API will appear here.

Before version 2.2 of the Simulator, you had to create the "fake-sdcard" directory manually for this to work. From 2.2 onwards, the "fake-sdcard" directory is created for you automatically.

Also from version 2.2 onwards, if you're running the Simulator from the command line you can define a different directory by passing the --storage-path option.

Note: this is mostly meant for API testing. Music will be read not from the "fake-sdcard" directory, but from your system's ~/Music, pictures from ~/Pictures, videos from ~/Videos. Downloads go to ~/Downloads. user-dirs.dirs redefinitions are supported.

Limitations of the Simulator

Note that the Firefox OS Simulator isn't a perfect simulation.

Hardware limitations

Apart from screen size, the Simulator does not simulate the hardware limitations of a Firefox OS device such as available memory or CPU speed.

Audio/video codecs

The following codecs depend on hardware-accelerated decoding and are therefore not yet supported:

  • MP3
  • AAC
  • H.264 (MP4)
  • WebM

This means it isn't possible to use the Simulator to test video playback in apps and on websites like Youtube that rely on these codecs.

Unsupported APIs

Certain APIs that work on the device won't work on the Simulator, generally because the supporting hardware is not available on the desktop. We've implemented simulations for some APIs such as geolocation, and expect to add more in future releases. However, at the moment the following APIs are not supported. Using them might throw errors or just return incorrect results:

Getting help

If you have a question, try asking us on the dev-developer-tools mailing list or on #devtools on

How to enable verbose logging

You can see messages logged from your app in the Web Console, which you can attach to your app using the WebIDE. If you want to catch early messages happening during app startup, before the console gets connected and working, you can enable verbose logging in the Simulator.

Visit about:config and create a new preference. The preference name is different for each version of the Simulator:

  • for Firefox OS 1.3
  • for Firefox OS 1.2

Set it to the string value "all", and disable, then reenable, the add-on in the Add-on Manager. Now extra messages about the Simulator's operation will appear in the Browser Console.

Building the Simulator

If you'd like to test patches to the Gecko or Gaia code that the Simulator contains, you may be interested in modifying the simulator to use a custom Gecko build or Gaia profile. Alternatively, you can build a new Simulator from your Gecko checkout.


Different ways to run Gaia — including a list of them in order of ease for running and reverse order of closeness to the shipped product (according to it, Firefox OS Simulator is easiest but furthest).