--- a/browser/components/tabview/favicons.js
+++ b/browser/components/tabview/favicons.js
@@ -18,17 +18,19 @@ var FavIcons = {
   },
 
   // Lazy getter for pref browser.chrome.favicons.
   get _prefFavicons() {
     delete this._prefFavicons;
     this._prefFavicons = Services.prefs.getBoolPref(this.PREF_CHROME_FAVICONS);
   },
 
-  get defaultFavicon() this._favIconService.defaultFavicon.spec,
+  get defaultFavicon() {
+    return this._favIconService.defaultFavicon.spec;
+  },
 
   init: function FavIcons_init() {
     XPCOMUtils.defineLazyServiceGetter(this, "_favIconService",
       "@mozilla.org/browser/favicon-service;1", "nsIFaviconService");
 
     Services.prefs.addObserver(this.PREF_CHROME_SITE_ICONS, this, false);
     Services.prefs.addObserver(this.PREF_CHROME_FAVICONS, this, false);
   },

--- a/browser/components/tabview/groupitems.js
+++ b/browser/components/tabview/groupitems.js
@@ -1278,17 +1278,17 @@ GroupItem.prototype = Utils.extend(new I
   _freezeItemSize: function GroupItem__freezeItemSize(itemCount) {
     let data = this._frozenItemSizeData;
 
     if (!data.lastItemCount) {
       let self = this;
       data.lastItemCount = itemCount;
 
       // unfreeze item size when tabview is hidden
-      data.onTabViewHidden = function () self._unfreezeItemSize();
+      data.onTabViewHidden = () => self._unfreezeItemSize();
       window.addEventListener('tabviewhidden', data.onTabViewHidden, false);
 
       // we don't need to observe mouse movement when expanded because the
       // tray is closed when we leave it and collapse causes unfreezing
       if (!self.expanded) {
         // unfreeze item size when cursor is moved out of group bounds
         data.onMouseMove = function (e) {
           let cursor = new Point(e.pageX, e.pageY);
@@ -1835,32 +1835,32 @@ GroupItem.prototype = Utils.extend(new I
 
   // ----------
   // Function: reorderTabItemsBasedOnTabOrder
   // Reorders the tabs in a groupItem based on the arrangment of the tabs
   // shown in the tab bar. It does it by sorting the children
   // of the groupItem by the positions of their respective tabs in the
   // tab bar.
   reorderTabItemsBasedOnTabOrder: function GroupItem_reorderTabItemsBasedOnTabOrder() {
-    this._children.sort(function(a,b) a.tab._tPos - b.tab._tPos);
+    this._children.sort((a,b) => a.tab._tPos - b.tab._tPos);
 
     this.arrange({animate: false});
     // this.arrange calls this.save for us
   },
 
   // Function: reorderTabsBasedOnTabItemOrder
   // Reorders the tabs in the tab bar based on the arrangment of the tabs
   // shown in the groupItem.
   reorderTabsBasedOnTabItemOrder: function GroupItem_reorderTabsBasedOnTabItemOrder() {
     let indices;
-    let tabs = this._children.map(function (tabItem) tabItem.tab);
+    let tabs = this._children.map(tabItem => tabItem.tab);
 
     tabs.forEach(function (tab, index) {
       if (!indices)
-        indices = tabs.map(function (tab) tab._tPos);
+        indices = tabs.map(tab => tab._tPos);
 
       let start = index ? indices[index - 1] + 1 : 0;
       let end = index + 1 < indices.length ? indices[index + 1] - 1 : Infinity;
       let targetRange = new Range(start, end);
 
       if (!targetRange.contains(tab._tPos)) {
         gBrowser.moveTabTo(tab, start);
         indices = null;
@@ -2417,17 +2417,17 @@ var GroupItems = {
   // Hides and shows tabs in the tab bar based on the active groupItem
   _updateTabBar: function GroupItems__updateTabBar() {
     if (!window.UI)
       return; // called too soon
 
     Utils.assert(this._activeGroupItem, "There must be something to show in the tab bar!");
 
     let tabItems = this._activeGroupItem._children;
-    gBrowser.showOnlyTheseTabs(tabItems.map(function(item) item.tab));
+    gBrowser.showOnlyTheseTabs(tabItems.map(item => item.tab));
   },
 
   // ----------
   // Function: updateActiveGroupItemAndTabBar
   // Sets active TabItem and GroupItem, and updates tab bar appropriately.
   // Parameters:
   // tabItem - the tab item
   // options - is passed to UI.setActive() directly

--- a/browser/components/tabview/iq.js
+++ b/browser/components/tabview/iq.js
@@ -641,17 +641,19 @@ iQClass.prototype = {
     return this;
   },
 
   // ----------
   // Function: bind
   // Binds the given function to the given event type. Also wraps the function
   // in a try/catch block that does a Utils.log on any errors.
   bind: function iQClass_bind(type, func) {
-    let handler = function(event) func.apply(this, [event]);
+    let handler = function(event) {
+      return func.apply(this, [event]);
+    };
 
     for (let i = 0; this[i] != null; i++) {
       let elem = this[i];
       if (!elem.iQEventData)
         elem.iQEventData = {};
 
       if (!elem.iQEventData[type])
         elem.iQEventData[type] = [];

--- a/browser/components/tabview/modules/utils.jsm
+++ b/browser/components/tabview/modules/utils.jsm
@@ -79,35 +79,43 @@ Rect.prototype = {
   // ----------
   // Function: toString
   // Prints [Rect (left,top,width,height)] for debug use
   toString: function Rect_toString() {
     return "[Rect (" + this.left + "," + this.top + "," +
             this.width + "," + this.height + ")]";
   },
 
-  get right() this.left + this.width,
+  get right() {
+    return this.left + this.width;
+  },
   set right(value) {
     this.width = value - this.left;
   },
 
-  get bottom() this.top + this.height,
+  get bottom() {
+    return this.top + this.height;
+  },
   set bottom(value) {
     this.height = value - this.top;
   },
 
   // ----------
   // Variable: xRange
   // Gives you a new <Range> for the horizontal dimension.
-  get xRange() new Range(this.left, this.right),
+  get xRange() {
+    return new Range(this.left, this.right);
+  },
 
   // ----------
   // Variable: yRange
   // Gives you a new <Range> for the vertical dimension.
-  get yRange() new Range(this.top, this.bottom),
+  get yRange() {
+    return new Range(this.top, this.bottom);
+  },
 
   // ----------
   // Function: intersects
   // Returns true if this rectangle intersects the given <Rect>.
   intersects: function Rect_intersects(rect) {
     return (rect.right > this.left &&
             rect.left < this.right &&
             rect.bottom > this.top &&
@@ -686,17 +694,17 @@ this.Utils = {
     }
     return value;
   },
 
   // ----------
   // Function: merge
   // Merge two array-like objects into the first and return it.
   merge: function Utils_merge(first, second) {
-    Array.forEach(second, function(el) Array.push(first, el));
+    Array.forEach(second, el => Array.push(first, el));
     return first;
   },
 
   // ----------
   // Function: extend
   // Pass several objects in and it will combine them all into the first object and return it.
   extend: function Utils_extend() {
 

--- a/browser/components/tabview/tabview.js
+++ b/browser/components/tabview/tabview.js
@@ -18,18 +18,22 @@ XPCOMUtils.defineLazyGetter(this, "tabvi
   return Services.strings.
     createBundle("chrome://browser/locale/tabview.properties");
 });
 XPCOMUtils.defineLazyGetter(this, "tabbrowserBundle", function() {
   return Services.strings.
     createBundle("chrome://browser/locale/tabbrowser.properties");
 });
 
-function tabviewString(name) tabviewBundle.GetStringFromName('tabview.' + name);
-function tabbrowserString(name) tabbrowserBundle.GetStringFromName(name);
+function tabviewString(name) {
+  return tabviewBundle.GetStringFromName('tabview.' + name);
+}
+function tabbrowserString(name) {
+  return tabbrowserBundle.GetStringFromName(name);
+}
 
 XPCOMUtils.defineLazyGetter(this, "gPrefBranch", function() {
   return Services.prefs.getBranch("browser.panorama.");
 });
 
 XPCOMUtils.defineLazyModuleGetter(this, "gPageThumbnails",
   "resource://gre/modules/PageThumbs.jsm", "PageThumbs");
 
@@ -47,17 +51,17 @@ var AllTabs = {
     move:         "TabMove",
     open:         "TabOpen",
     select:       "TabSelect",
     pinned:       "TabPinned",
     unpinned:     "TabUnpinned"
   },
 
   get tabs() {
-    return Array.filter(gBrowser.tabs, function (tab) Utils.isValidXULTab(tab));
+    return Array.filter(gBrowser.tabs, tab => Utils.isValidXULTab(tab));
   },
 
   register: function AllTabs_register(eventName, callback) {
     gBrowser.tabContainer.addEventListener(this._events[eventName], callback, false);
   },
 
   unregister: function AllTabs_unregister(eventName, callback) {
     gBrowser.tabContainer.removeEventListener(this._events[eventName], callback, false);

--- a/browser/components/tabview/trench.js
+++ b/browser/components/tabview/trench.js
@@ -82,17 +82,19 @@ Trench.prototype = {
     return "[Trench " + this.edge + " " + this.type +
            (this.parentItem ? " (" + this.parentItem + ")" : "") +
            "]";
   },
 
   //----------
   // Variable: radius
   // (integer) radius is how far away we should snap from
-  get radius() this.customRadius || Trenches.defaultRadius,
+  get radius() {
+    return this.customRadius || Trenches.defaultRadius;
+  },
 
   setParentItem: function Trench_setParentItem(item) {
     if (!item.isAnItem) {
       Utils.assert(false, "parentItem must be an Item");
       return false;
     }
     this.parentItem = item;
     return true;