_ff-addon-snippet-RedirectChannel: This snippet uses observer service and http-on-modify-request to listen to if google.com is loaded and it redirects to bing.com. A Firefox addon snippet that must be run from privelaged scope.

_ff-addon-snippet-RedirectChannel.js

Cu.import('resource://gre/modules/Services.jsm');

var httpRequestObserver = 
{
    observe: function(subject, topic, data)
    {
        var httpChannel, requestURL;

        if (topic == 'http-on-modify-request') {
            httpChannel = subject.QueryInterface(Ci.nsIHttpChannel);
            requestURL = httpChannel.URI.spec;

            if (/google\.com/.test(requestURL)) {
                httpChannel.redirectTo(Services.io.newURI('http://www.bing.com/', undefined, undefined)); //redirectTo requires at least Firefox 20
            }

            return;
        }
    }
};

Services.obs.addObserver(httpRequestObserver, 'http-on-modify-request', false);
//Services.obs.removeObserver(httpRequestObserver, "http-on-modify-request", false);

README

Rev1

This method requires minimum Firefox version of 20

Rev2

I have not created this yet but I'm thinking of a version that works even below Firefox 20, we can cancel the current channel (httpChannel.cancel(Cr.NS_BINDING_ABORTED);) and/or replace it with another channel.

Rev3

• Renamed file name to snippet from demo

rename this to _snippet or _scratchpad