Example of defining content policies.

policy.js

// For details see:
// https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsIContentPolicy
// https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsICategoryManager

const { Ci } = require('chrome')
const xpcom = require('api-utils/xpcom')

const policy = {
  contractID: '@lduros.net/PreventImage-policy',
  name: 'Implements content policy to prevent images from being loaded',
  QueryInterface: xpcom.utils.generateQI([ 'nsIContentPolicy' ]),
  create: function create() {
    console.log('create an instance')
    return policy // Make it a singleton
  },
  shouldLoad: function (contType, contLoc, reqOrig, ctx, typeGuess, extra) {
    console.log('checking type')
    if (contType === 3) {
      console.log('detected an image')
      return Ci.nsIContentPolicy.REJECT_OTHER
    } else {
      return Ci.nsIContentPolicy.ACCEPT
    }
  },
  shouldProcess: function (contType, contLoc, reqOrig, ctx, mimeType, extra) {
    return Ci.nsIContentPolicy.ACCEPT
  }
}
xpcom.register(policy)

xpcom.utils.categoryManager.addCategoryEntry('content-policy', policy.name, policy.contractID, false, true)