Examine source code of MetricRules!

{
  "manifest_version": 2,
  "name": "Metric Conversion",
  "version": "0.1",
  "description": "Converts some freedom units (imperial) to metric.",
  "icons": {
    "48": "icon48.png",
    "128": "icon128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*/*"
      ],
      "js": [
        "content_script.js"
      ],
      "run_at": "document_end"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{20766eb9-4193-4f2e-a23e-9c8766a8e5d5}"
    }
  }
}