Examine source code of 1 2 key for tab switch

{
  "developer": {
    "name": "Dmitriy Zhuravlev",
    "url": "https://dmitriyzhuravlev.ru/"
  },
  "manifest_version": 2,
  "name": "1 2 single key shorcut for tab switching",
  "version": "1.2",
  "description": "Now you can go to the previous or to next tab via singe keybord keys 1 and 2. Switch your Firefox tabs like in old Opera 12 Presto!",
  "icons": {
    "48": "icons/icon.png"
  },
  "permissions": [
    "<all_urls>",
    "tabs"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ]
}