Examine source code of Open in Steam

{
  "manifest_version": 2,
  "name": "Open in Steam",
  "version": "1.0",
  "description": "Adds a button to the address bar for instant switching to the Steam client.",
  "icons": {
    "16": "steam.svg"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.steampowered.com/",
        "*://steamcommunity.com/"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "openinsteam@maclews.eu"
    }
  },
  "page_action": {
    "default_icon": "steam.svg",
    "default_title": "Open in Steam"
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "permissions": [
    "tabs"
  ]
}