Examine source code of DiscountDashboardBells

{
  "manifest_version": 2,
  "name": "DiscountDashboardBells",
  "version": "0.6",
  "description": "Adds bells to the top of the tumblr dashboard.",
  "icons": {
    "48": "icons/blue_bell_48_48_bright.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.tumblr.com/*"
      ],
      "js": [
        "simple_body_bells.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{3671db59-f082-4e02-8af4-1539b66cb423}"
    }
  }
}