Examine source code of Keep Container

{
  "manifest_version": 2,
  "name": "Keep Container",
  "version": "0.3",
  "description": "Opens new tabs in the same container as the previous used tab. To open a new tab in the default container, simply create a new tab while on the new tab page.",
  "permissions": [
    "tabs",
    "contextualIdentities",
    "cookies"
  ],
  "background": {
    "scripts": [
      "keepcontainer.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{7e896097-1065-41a2-84a9-fe96f33832a1}"
    }
  }
}