Examine source code of TweetDeck+

{
  "manifest_version": 2,
  "name": "TweetDeck+",
  "version": "0.1.5resigned1",
  "description": "Add buttons and keyboard shortcuts to tweetdeck for clear all tweets or column tweets ",
  "icons": {
    "48": "icons/tweetdeck+-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.tweetdeck.twitter.com/*"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "web_accessible_resources": [
    "TweetDeck_Clear.js",
    "280Characters.js"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{31efcce8-ff59-4910-93c4-841280c0ccfd}"
    }
  }
}