Examine source code of Pressd.

{
  "manifest_version": 2,
  "name": "Pressd.",
  "version": "1.1",
  "description": "Get the history of your keystrokes for the last 1 hour of any page!",
  "permissions": [
    "storage",
    "activeTab",
    "tabs"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icons/icon16.png",
      "48": "icons/icon48.png",
      "128": "icons/icon128.png"
    }
  },
  "options_ui": {
    "page": "popup.html",
    "open_in_tab": false
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{9b3154af-fd8c-4bf4-bd8f-ed9754b9b96a}"
    }
  }
}