Auto Private, https://addons.mozilla.org/addon/auto-private/

autoprivate.js

// Auto Private, https://addons.mozilla.org/addon/auto-private/ (code from v.1.5 + some modifications)
var prefs = {
   name: ["extensions.autoprivate.domains", "extensions.autoprivate.parts"],
   domains: "",
   parts: "",
   init: function() {
      var name1 = this.name[0];
	  var name2 = this.name[1];
      var defaultBranch = Services.prefs.getDefaultBranch("");
      if(defaultBranch.getPrefType(name1) != defaultBranch.PREF_STRING)
         defaultBranch.deleteBranch(name1);
      if(defaultBranch.getPrefType(name2) != defaultBranch.PREF_STRING)
         defaultBranch.deleteBranch(name2);
      defaultBranch.setCharPref(name1, "");
	  defaultBranch.setCharPref(name2, "");
      this.readBlacklists();
      Services.prefs.addObserver(name1, this, false);
	  Services.prefs.addObserver(name2, this, false);
   },
   destroy: function() {
      Services.prefs.removeObserver(this.name[0], this);
	  Services.prefs.removeObserver(this.name[1], this);
   },
   observe: function(subject, topic, pName) {
      this.readBlacklists();
   },
   readBlacklists: function() {
	  var domains = Services.prefs.getComplexValue(this.name[0], Ci.nsISupportsString).data;
	  if(/\//.test(domains))
		Services.prefs.setCharPref(this.name[0], domains.replace(/(\w*:\/{2,4})?([^/;]+)\/?.*?(;|$)/g, "$2$3"));
      this.domains = Services.prefs.getComplexValue(this.name[0], Ci.nsISupportsString).data;
	  this.parts = Services.prefs.getComplexValue(this.name[1], Ci.nsISupportsString).data;
   }
};

var httpRequestObserver =
{
  observe: function(subject, topic, data)
  {
   if(subject instanceof Ci.nsIHttpChannel && subject.URI){
	   var eTldService = Components
		.classes["@mozilla.org/network/effective-tld-service;1"]
		.getService(Ci.nsIEffectiveTLDService);
	   var tld = eTldService.getPublicSuffix(subject.URI);
	   var base = eTldService.getBaseDomain(subject.URI);
	   var prefix = subject.URI.host.replace(base, "");
	   var domains = prefs.domains.replace(/(.+?)\.tld(;|$)/g, "$1." + tld + "$2").replace(/(^|;)\./g, "$1" + prefix);
	   var parts = new RegExp("^(" + prefs.parts.replace(/(.+?)\.tld(\/.*?|\*?)(;|$)/g, "$1." + tld + "$2$3").replace(/([\w*]*:\/{2,4})\./g, "$1" + prefix).replace(/([?^$+{}[\]/.\\])/g, "\\$1").replace(/\*/g, ".*").replace(/^;|;$/g, "").replace(/;/g, "|") + ")$", "i");
	   if(
	      domains.split(";").indexOf(subject.URI.host) != -1
		  || parts.test(subject.URI.spec)
	   ) {
		  var httpChannel = subject;
		  var goodies = loadContextGoodies(httpChannel);
		  if(goodies) {
			 var loadContext = goodies.loadContext;
			 if(loadContext.usePrivateBrowsing) // Already private
				return;
			 var window = goodies.aDOMWindow;
			 var tab = goodies.aTab;
			 loadContext.usePrivateBrowsing = true;
			 if("privateTab" in window) {
				tab.dispatchEvent(new window.UIEvent("PrivateTab:PrivateChanged", {
				   bubbles: true,
				   cancelable: false,
				   detail: 1,
				   view: window
				}));
			 }
			 else {
				tab.setAttribute("autoPrivate-isPrivate", "true");
				var s = tab.style;
				s.textDecoration = "underline";
				s.textDecorationColor = s.MozTextDecorationColor = "red";
				s.textDecorationStyle = s.MozTextDecorationStyle = "dashed";
			 }
			 httpChannel.redirectTo(subject.URI.clone());
		  }
	   }
	}
  },

  register: function()
  {
    Services.obs.addObserver(this, "http-on-modify-request", false);
  },
  unregister: function()
  {
    Services.obs.removeObserver(this, "http-on-modify-request");
  }
};

function loadContextGoodies(httpChannel) {
    var loadContext;
    try {
        var interfaceRequestor = httpChannel.notificationCallbacks.QueryInterface(Ci.nsIInterfaceRequestor);
        try {
            loadContext = interfaceRequestor.getInterface(Ci.nsILoadContext);
        } catch (ex) {
            try {
                loadContext = subject.loadGroup.notificationCallbacks.getInterface(Ci.nsILoadContext);
            } catch (ex2) {}
        }
    } catch (ex0) {}

    if (!loadContext) {
        return null;
    } else {
        var contentWindow = loadContext.associatedWindow;
        if (!contentWindow) {
            return null;
        } else {
            //Services.console.logStringMessage("[Auto Private] " + contentWindow.location.href + "\n" + httpChannel.URI.spec);
            if (
                contentWindow != contentWindow.top // Ignore frames
                || contentWindow instanceof Ci.nsIDOMChromeWindow // Doesn't looks like content window
                || !(httpChannel instanceof Ci.nsIRequest) // Grant access to nsIRequest.loadFlags
                || !(httpChannel.loadFlags & httpChannel.LOAD_DOCUMENT_URI)
            )
                return null;
            var aDOMWindow = contentWindow.QueryInterface(Ci.nsIInterfaceRequestor)
                .getInterface(Ci.nsIWebNavigation)
                .QueryInterface(Ci.nsIDocShellTreeItem)
                .rootTreeItem
                .QueryInterface(Ci.nsIInterfaceRequestor)
                .getInterface(Ci.nsIDOMWindow);
            var gBrowser = aDOMWindow.gBrowser;
            if (!gBrowser)
                return null;
            var aTab = gBrowser._getTabForContentWindow(contentWindow);
            if (!aTab)
                return null;
            return {
                loadContext: loadContext,
                aDOMWindow: aDOMWindow,
                gBrowser: gBrowser,
                aTab: aTab,
                contentWindow: contentWindow
            };
        }
    }
}