Examine source code of Timebox Timer

{
  "manifest_version": 2,
  "name": "Timebox Timer",
  "version": "1.5.1",
  "description": "Displays a timer as an overlay on the web page currently open in Firefox",
  "icons": {
    "16": "icons/16.png",
    "32": "icons/32.png",
    "48": "icons/48.png",
    "64": "icons/64.png",
    "96": "icons/96.png",
    "128": "icons/128.png"
  },
  "permissions": [
    "activeTab",
    "storage"
  ],
  "browser_action": {
    "default_popup": "popup/start-timer.html",
    "default_title": "Start timebox timer",
    "default_icon": {
      "16": "icons/16.png",
      "32": "icons/32.png",
      "48": "icons/48.png",
      "64": "icons/64.png",
      "96": "icons/96.png",
      "128": "icons/128.png"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{dda4c8a1-2cff-42f2-8677-cf0655d22cb5}"
    }
  }
}