Examine source code of Right click text counter

{
  "manifest_version": 2,
  "name": "Right Click Text Counter",
  "description": "Display text length on context menu(right click menu)",
  "version": "1.0",
  "applications": {
    "gecko": {
      "strict_min_version": "60.0",
      "id": "{b9cbd3b5-4357-4ebe-a1e4-232b7efc2034}"
    }
  },
  "content_scripts": [
    {
      "all_frames": true,
      "matches": [
        "http://*/*",
        "https://*/*",
        "ftp://*/*"
      ],
      "js": [
        "src/content.js"
      ]
    }
  ],
  "background": {
    "scripts": [
      "src/background.js"
    ]
  },
  "permissions": [
    "<all_urls>",
    "contextMenus"
  ],
  "icons": {
    "96": "icons/icon_96.png"
  }
}