onLocationChange: function(aBrowser, aWebProgress, aRequest, aLocation, aFlags) {

    if(aRequest != null) {

        // suspend early, because that becomes impossible later
        aRequest.suspend();

        // check if it is HTTPS (no HTTP, no about:, etc.)
        if(aLocation.scheme == "https") {

            var whateverIWantToDo = {
                notify: function(timer) {

                    try {
                        // for example I'm interested in the validation result:
                        let secUI = aBrowser.securityUI;
                        secUI.QueryInterface(Components.interfaces.nsISSLStatusProvider);
                        let status = secUI.SSLStatus;

                        // do something what you want here
                        // and do not forget to resume the request at any point. otherwise that could lead to problems
                        aRequest.resume();
                        //....
                        // the reason I suspend the request at all:
                        // I do not want to completely load the page until I know if the certificate is valid
                    }
                    catch(err) {
                        // maybe it's too early to access the SSLStatus when the event handler kicks in.
                        // in this case an error will be thrown which is caught here
                        // wait 10 ms and try it again, usually that works! (I know, it's a bit hacky)
                        var timer = Components.classes["@mozilla.org/timer;1"].createInstance(Components.interfaces.nsITimer);
                        timer.initWithCallback(whateverIWantToDo, 10, Components.interfaces.nsITimer.TYPE_ONE_SHOT);
                    }

                }
            }

            whateverIWantToDo.notify();

        } else {
            // continue normally when it's not a HTTPS request, we want to intercept
            aRequest.resume();
        }
    }
}