FireFox Plugin

main.js

// AssetErrorController.getErrors() needs to be exposed to either the window or document scope on pages
// i.e. javascript:alert(window.foo); should work with the plugin loaded and return an object/hash back

var events = require("sdk/system/events");
var { Ci, Cu } = require("chrome");

// Store errors here
var AssetErrorController = new function() {
    var errors = {};

    this.put = function(url, status) {
        if(errors[url] == undefined) {
            errors[url] = [];
        }

        errors[url].push(status);
    };

    this.getErrors = function() {
        return errors;
    }
};

// Catch errors
events.on("http-on-examine-response", function (event) {
    try {
        event.subject.QueryInterface(Ci.nsIHttpChannel);

        // If the request failed, but wasn't a redirect 3xx (redirects are not a success)
        var status = event.subject.responseStatus;
        var url = String(event.subject.URI.spec);

        if(
            // Didn't load
            (event.subject.requestSucceeded == false && !(status >= 300 && status < 400)) ||

            // Internal address
            (/^https?:\/\/((127|10)\.[0-9]{1,3}\.)|(172\.((1[6-9])|(2[0-9])|(3[0-1]))\.)|(192\.168\.)\.[0-9]{1,3}\.[0-9]{1,3}/.test(url))
        ) {
            AssetErrorController.put(url, status);
        }
    } catch(err) {
        console.log("ERROR: Capture error: " + err);
    }
}, true);

exports.main = function(options, callbacks) {
    events.on("content-document-global-created", function(event) {

        // This throws a security error
        // Security wrapper denied access to property undefined on privileged Javascript object.
        // Support for exposing privileged objects to untrusted content via __exposedProps__ is being gradually removed -
        // use WebIDL bindings or Components.utils.cloneInto instead.
        // Note that only the first denied property access from a given global object will be reported.
        //Cu.exportFunction(AssetErrorController.getErrors, event.subject.window, {defineAs: "bar"});

        // Throws a permission denied error
        // Error: Permission denied to pass a Function via structured clone
        //event.subject.window.bar = Cu.cloneInto(AssetErrorController, event.subject.window);
        //event.subject.window.bar = Cu.cloneInto(AssetErrorController.getErrors, event.subject.window);

        // Does not work without any errors
        //event.subject.window.bar = Cu.cloneInto(AssetErrorController, event.subject.window, {cloneFunctions: true});
        //event.subject.window.bar1 = Cu.cloneInto(AssetErrorController.getErrors, event.subject.window, {cloneFunctions: true});
        //event.subject.window.bar2 = Cu.cloneInto(AssetErrorController.getErrors, event.subject, {cloneFunctions: true});
        //event.subject.window.bar3 = Cu.cloneInto(AssetErrorController.getErrors, event, {cloneFunctions: true});

        // This works
        Cu.createObjectIn(event.subject.window, {defineAs: "foo"});
    }, true);
};