In Firefox you need to use XPCOM component called nsIHTTPChannel. This script below will block abort all rqeuests to google. The channel is opened but aborted before opening connection to server.
var {classes: Cc, results: Cr, utils: Cu} = Components;
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)) {
httpChanel.cancel(Cr.NS_BINDING_ABORTED);
}
return;
}
}
};
Services.obs.addObserver(httpRequestObserver, 'http-on-modify-request', false);
//Services.obs.removeObserver(httpRequestObserver, "http-on-modify-request", false); //run this to remove observer