Examine source code of LegacySupport

{
  "manifest_version": 2,
  "name": "LegacySupport",
  "version": "1.0",
  "description": "Support legacy browsers while deploying Firefox in your organization.",
  "permissions": [
    "<all_urls>"
  ],
  "icons": {
    "48": "icon.png"
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "background": {
    "scripts": [
      "popup.js"
    ],
    "persistent": false
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{25621062-ea7f-432d-ba6e-a65ac786152e}"
    }
  }
}