Examine source code of OldMapsOnline

{
  "manifest_version": 2,
  "name": "Georeferencer",
  "description": "Automatically find maps on web pages you visit and add them to Georeferencer mapping platform.",
  "version": "1.2.2resigned1",
  "icons": {
    "16": "images/icon16.png",
    "32": "images/icon32.png",
    "38": "images/icon38.png",
    "48": "images/icon48.png",
    "64": "images/icon64.png",
    "128": "images/icon128.png",
    "256": "images/icon256.png"
  },
  "background": {
    "scripts": [
      "extension.js"
    ]
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "images/icon16.png",
      "32": "images/icon32.png",
      "38": "images/icon38.png",
      "48": "images/icon48.png",
      "64": "images/icon64.png",
      "128": "images/icon128.png",
      "256": "images/icon256.png"
    }
  },
  "permissions": [
    "webRequest",
    "tabs",
    "*://*/*"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{a84f601c-824c-4309-95e0-b8df93b459b7}"
    }
  }
}