Examine source code of Header Editor FG

{
  "manifest_version": 2,
  "name": "Header Editor",
  "version": "1.3",
  "description": "A Mozilla extension to add, modify, and remove HTTP headers.",
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>",
    "storage"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_title": "Header Editor"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{ee543fec-4af0-44e7-9d1b-e826a5a67d61}"
    }
  }
}