Examine source code of rmblue

{
  "manifest_version": 2,
  "name": "rmblue",
  "version": "1.0.0",
  "description": "Removes the Twitter Blue sign-up button from the sidebar in the Twitter Web App.",
  "icons": {
    "48": "icons/rmblue-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.twitter.com/*"
      ],
      "js": [
        "rmblue.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "rmblue@rmblue"
    }
  }
}