Examine source code of Keep Working!

{
  "manifest_version": 2,
  "name": "Keep Working!",
  "version": "1.1.1",
  "description": "Limit the time you can pass on websites you choose!",
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "sudo@keep_working_firefox"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "permissions": [
    "storage",
    "tabs"
  ],
  "browser_action": {
    "default_icon": "sledgehammer-32.png",
    "default_title": "Keep Working!",
    "default_popup": "popup/pop_up.html"
  },
  "icons": {
    "32": "sledgehammer-32.png",
    "48": "sledgehammer-48.png",
    "64": "sledgehammer-64.png"
  }
}