Examine source code of Device Emulator

{
  "browser_action": {
    "browser_style": true,
    "default_title": "Mobile View Tester",
    "default_popup": "window.html",
    "default_icon": {
      "64": "icons/logo_64.png"
    }
  },
  "icons": {
    "32": "icons/logo_32.png",
    "64": "icons/logo_64.png",
    "128": "icons/logo_128.png"
  },
  "description": "Test the look of a website on different mobile devices",
  "homepage_url": "https://github.com/nabendu82/Addons",
  "manifest_version": 2,
  "name": "Mobile View Tester",
  "version": "1.0",
  "browser_specific_settings": {
    "gecko": {
      "id": "{b6712157-8258-4a5e-ab89-0f295cf96dd4}"
    }
  }
}