Firefox Module barebones view of the required code for a ContentPolicy implementation. nsIContentPolicy

ContentPolicy.jsm

Components.utils.import("resource://gre/modules/XPCOMUtils.jsm");
var EXPORTED_SYMBOLS = ["ContentPolicy"];
var ContentPolicy = {
    classDescription: "Third Party Filtering",      //some description
    classID: Components.ID("dabef210-7f6b-11e0-b278-0800200c9a66"),  //UUID. Generated using UUID tool.
    contractID: "@${app_name}/filter/thirdparty;1", //structure:  "@${app_name}/${generic_descriptor}/${name};1"
    QueryInterface: XPCOMUtils.generateQI(
        [  Components.interfaces.nsIContentPolicy,
           Components.interfaces.nsIObserver, 
    	   Components.interfaces.nsIChannelEventSink, 
           Components.interfaces.nsIFactory  ] ), //all the interfaces the module implements. IMPORTANT!!!!!
    createInstance: function(outer, iid){
    	if (outer) throw Components.results.NS_ERROR_NO_AGGREGATION;
    	return this.QueryInterface(iid);
    }
}

ContentPolicy.jss

//////MAIN METHOD FOR FILTERING//////
/**
* NOTE!: DO NOT access properties, of any sort, on any DOM object, from here
* without using XPCNativeWrapper (either explicitly or implicitly)
* 
* @param contentType : The type of the content requested, one of the following values: 
*       "OTHER", "SCRIPT", "IMAGE", "STYLESHEET", "OBJECT", "DOCUMENT", "SUBDOCUMENT", "REFRESH", 
*       "XBL", "PING", "XMLHTTPREQUEST", "OBJECT_SUBREQUEST", "DTD", "FONT", "MEDIA"
* @param location : The location (nsIURI) of the content to be loaded. Cannot be null.
* @param origin (optional) : The location (nsIURI) of the requesting resource. Can be null.
* @param context (optional) : The nsIDOMWindow or nsIDOMNode that initiated the request. Can be null.
*                               The Context is the -new- tab/window if a tab/window is opened by user action.
* @param mimeGuess (optional) : A guess at the content's mime Type. 
* @param extra (optional) : Extra variable for non-Gecko callers to pass data to callees.
* @return 1,-1 Indicating Allow, or Block, the request.
*/
shouldLoad: function ContentPolicy_shouldLoad(contentType, location, origin, context, mimeTypeGuess, extra) {
    if(should_block) return -1;
    else return 1;
}
shouldProcess: function(){
    return 1;
}
onChannelRedirect: function ContentPolicy_onChannelRedirect(oldChannel, newChannel, flags){
    if(want_to_block) throw Components.results.NS_BASE_STREAM_WOULD_BLOCK;
    else { /*do nothing*/ }
}
asyncOnChannelRedirect: function(oldChannel, newChannel, flags, callback){
    this.onChannelRedirect(oldChannel, newChannel, flags);
    callback.onRedirectVerifyCallback(Components.results.NS_OK);
}

ContentPolicy1.jss

///FOR REGISTERING THE MODULE. REQUIRED IF THE BROWSER WILL BE USING YOUR MODULE
var cr = Components.manager.QueryInterface(Components.interfaces.nsIComponentRegistrar);
try{
    cr.registerFactory(this.classID, this.classDescription, this.contractID, ContentPolicy);
}catch(e){}

///REGISTERING THE MODULE TO CERTAIN BROWSER EVENT LISTENER/OBSERVER
var cm = Components.classes["@mozilla.org/categorymanager;1"].getService(Components.interfaces.nsICategoryManager);
cm.addCategoryEntry("content-policy", this.classDescription, this.contractID, false, true);
cm.addCategoryEntry("net-channel-event-sinks", this.classDescription, this.contractID, false, true);
 
///ALSO good to later on remove the listeners
cm.deleteCategoryEntry("content-policy", this.classDescription, false);
cm.deleteCategoryEntry("net-channel-event-sinks", this.classDescription, false);