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

you can have a look at the source of those addons

https://addons.mozilla.org/en-us/firefox/addon/blocksite/?src=search https://addons.mozilla.org/en-us/firefox/addon/url-n-extension-blockune-bl/?src=search

or use service observer with nsIHTTPChannel for fast handling

const { Ci, Cu, Cc, Cr } = require('chrome'); //const {interfaces: Ci, utils: Cu, classes: Cc, results: Cr } = Components;
Cu.import('resource://gre/modules/Services.jsm');
Cu.import('resource://gre/modules/devtools/Console.jsm');

var observers = {
    'http-on-modify-request': {
        observe: function (aSubject, aTopic, aData) {
            console.info('http-on-modify-request: aSubject = ' + aSubject + ' | aTopic = ' + aTopic + ' | aData = ' + aData);
            var httpChannel = aSubject.QueryInterface(Ci.nsIHttpChannel);
            var requestUrl = httpChannel.URI.spec
            if (requestUrl.indexOf('google.com') > -1) {
               //httpChannel.cancel(Cr.NS_BINDING_ABORTED); //this aborts the load
               httpChannel.redirectTo(Services.io.newURI('data:text,url_blocked', null, null)); //can redirect with this line, if dont want to redirect and just block, then uncomment this line and comment out line above (line 17)
            }
        },
        reg: function () {
            Services.obs.addObserver(observers['http-on-modify-request'], 'http-on-modify-request', false);
        },
        unreg: function () {
            Services.obs.removeObserver(observers['http-on-modify-request'], 'http-on-modify-request');
        }
    }
};

To start observing

To start start obseving all requests do this (for example on startup of your addon)

for (var o in observers) {
    observers[o].reg();
}

To stop observing

Its important to stop observring (make sure to run this at least on shutdown of addon, you dont want to leave the observer registered for memory reasons)

for (var o in observers) {
    observers[o].unreg();
}

Full working example of the observer service to block/redirect urls: https://github.com/Noitidart/PortableTester/tree/block-urls

Answer 2

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?