Examine source code of Digital55 PlugIn-Gmail

{
  "description": "Allows to select & send customized design email template from your gmail.",
  "manifest_version": 2,
  "name": "Digital55-GmailPlugin",
  "version": "1.1resigned1",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/open-my-page-button",
  "icons": {
    "48": "icons/48x48.png"
  },
  "applications": {
    "gecko": {
      "id": "Gmail-Plugin@mozilla.org",
      "strict_min_version": "45.0"
    }
  },
  "background": {
    "page": "background.html"
  },
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*"
      ],
      "js": [
        "custom/content.js"
      ]
    }
  ],
  "permissions": [
    "alarms",
    "tabs",
    "webNavigation",
    "*://*.google.com/",
    "http://*/*",
    "https://*/*",
    "https://www.google.com/*",
    "http://www.google.com/*",
    "https://1pointapi2.psplhyd.com/*"
  ],
  "web_accessible_resources": [
    "custom/MyBundle.js"
  ],
  "browser_action": {
    "default_icon": "gmail_not_logged_in.png"
  }
}