Fixed back-to-top button on firefox. Fixes #153.

@@ -10,9 +10,12 @@ require('angular-ui-sortable');
 var ngApp = angular.module('bookStack', ['ngResource', 'ngAnimate', 'ngSanitize', 'ui.sortable']);
 
 // Global Event System
-var Events = {
-    listeners: {},
-    emit: function (eventName, eventData) {
+class Events {
+    constructor() {
+        this.listeners = {};
+    }
+
+    emit(eventName, eventData) {
         if (typeof this.listeners[eventName] === 'undefined') return this;
         var eventsToStart = this.listeners[eventName];
         for (let i = 0; i < eventsToStart.length; i++) {
@@ -20,14 +23,15 @@ var Events = {
             event(eventData);
         }
         return this;
-    },
-    listen: function (eventName, callback) {
+    }
+
+    listen(eventName, callback) {
         if (typeof this.listeners[eventName] === 'undefined') this.listeners[eventName] = [];
         this.listeners[eventName].push(callback);
         return this;
     }
 };
-window.Events = Events;
+window.Events = new Events();
 
 var services = require('./services')(ngApp, Events);
 var directives = require('./directives')(ngApp, Events);
@@ -38,14 +42,15 @@ var controllers = require('./controllers')(ngApp, Events);
 // Smooth scrolling
 jQuery.fn.smoothScrollTo = function () {
     if (this.length === 0) return;
-    $('body').animate({
+    let scrollElem = document.documentElement.scrollTop === 0 ?  document.body : document.documentElement;
+    $(scrollElem).animate({
         scrollTop: this.offset().top - 60 // Adjust to change final scroll position top margin
     }, 800); // Adjust to change animations speed (ms)
     return this;
 };
 
 // Making contains text expression not worry about casing
-$.expr[":"].contains = $.expr.createPseudo(function (arg) {
+jQuery.expr[":"].contains = $.expr.createPseudo(function (arg) {
     return function (elem) {
         return $(elem).text().toUpperCase().indexOf(arg.toUpperCase()) >= 0;
     };
@@ -95,13 +100,14 @@ $(function () {
     var scrollTop = document.getElementById('back-to-top');
     var scrollTopBreakpoint = 1200;
     window.addEventListener('scroll', function() {
-        if (!scrollTopShowing && document.body.scrollTop > scrollTopBreakpoint) {
+        let scrollTopPos = document.documentElement.scrollTop || document.body.scrollTop || 0;
+        if (!scrollTopShowing && scrollTopPos > scrollTopBreakpoint) {
             scrollTop.style.display = 'block';
             scrollTopShowing = true;
             setTimeout(() => {
                 scrollTop.style.opacity = 0.4;
             }, 1);
-        } else if (scrollTopShowing && document.body.scrollTop < scrollTopBreakpoint) {
+        } else if (scrollTopShowing && scrollTopPos < scrollTopBreakpoint) {
             scrollTop.style.opacity = 0;
             scrollTopShowing = false;
             setTimeout(() => {