Examine source code of m-wiki

{
  "manifest_version": 2,
  "name": "m-wiki",
  "version": "2.1resigned1",
  "description": "This extension redirects all visits to wikipedia to the mobile version of the site.",
  "content_scripts": [
    {
      "matches": [
        "http://*.wikipedia.org/wiki/*",
        "https://*.wikipedia.org/wiki/*"
      ],
      "js": [
        "mwiki.js"
      ],
      "run_at": "document_start"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "jid1-DcALCYFnBpWjCg@jetpack"
    }
  }
}