Examine source code of SessionResize

{
  "manifest_version": 2,
  "name": "SessionResize",
  "version": "1.0",
  "description": "Simply open a new window and close it to force Firefox to resize his window. Useful when restoring session on a tilling manager (such as i3).",
  "icons": {
    "48": "icons/border-48.png"
  },
  "background": {
    "scripts": [
      "background-script.js"
    ]
  }
}