Examine source code of Proton Switch

{
  "manifest_version": 2,
  "name": "Proton Switch",
  "version": "1.0.1",
  "description": "Easily switch between ProtonMail accounts with a single click.",
  "browser_action": {
    "default_popup": "popup/index.html",
    "default_icon": "icons/ps.png"
  },
  "icons": {
    "48": "icons/favicon-48x48.png",
    "96": "icons/favicon-96x96.png"
  },
  "permissions": [
    "*://account.proton.me/*",
    "storage"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{7afefd01-8c97-4c10-9850-849caf093589}"
    }
  }
}