Firefox extension: How to intercept the requested url conditionally and block it?

Question

In my Firefox extension I want to intercept the url that the browser is requesting and block the request entirely if some condition matches

How can I intercept URL b

Answer 1

An other possible solution :

Here is an other implementation as modules example from HTTPS-Everywhere

Init function :

  init: function() {
    // start observing all http requests
    Services.obs.addObserver(httpNowhere, "http-on-modify-request", false);
  },

Observer function :

observe: function(subject, topic, data) {
var request = subject.QueryInterface(Ci.nsIHttpChannel);
  if (topic == "http-on-modify-request") {
    if (request.URI.spec == "xxx.example.com") {
      request.redirectTo("yyy.example.com");
    }
    else {
      request.cancel(Components.results.NS_ERROR_ABORT);
    }
  }
},

Example addons :

HTTPS-Nowhere - https://github.com/cwilper/http-nowhere

HTTPS-Everywhere - https://github.com/EFForg/https-everywhere

Migrating your extension to chrome :

i answered your question for chrome in this page : Chrome Extension : How to intercept requested urls?