Add a `deviceorientation` comand to the Firefox command line.

orientation-command.js

/**
 * Running the following code in a Scratchpad running in the 'browser' context
 * will add a `deviceorientation` command to the Firefox command line.
 */ 
Components.utils.import("resource:///modules/devtools/gcli.jsm");

gcli.addCommand({
  name: 'deviceorientation',
  description: 'Fire a deviceorientation event',
  params: [{
      name: 'alpha',
      type: 'number'
    }, {
      name: 'beta',
      type: 'number'
    }, {
      name: 'gamma',
      type: 'number'
    }
  ],
  exec: function(args, context) {
    var win = context.environment.document.defaultView;
    var event = win.document.createEvent("DeviceOrientationEvent");

    event.initDeviceOrientationEvent('deviceorientation', true
                , true, args.alpha, args.gamma, args.omega, true);

    win.dispatchEvent(event);
  }
});

To run this, first enable chrome debugging:

1. Open the DevTools window with Web Developer > Toggle Tools.
2. Open the settings tab (the gear icon)
3. Check "Enable chrome debugging" under Advanced Settings
4. Restart Firefox (try the Restartless Restart addon)

Then open a Scratchpad:

1. Tools > Web Developer > Scratchpad
2. Set the scratchpad to "chrome" mode with Environment > Browser
3. Copy the above code into the scratchpad and run it (Cmd-R / Ctrl-R)

Try it out

1. Tools > Web Developer > Developer Toolbar
2. Navigate to Marine Compass demo
3. Type deviceorientation 30 40 -120 into the toolbar and press Enter:

Turn it into an addon