--- a/webapprt/content/downloads/downloads.js
+++ b/webapprt/content/downloads/downloads.js
@@ -573,17 +573,17 @@ DownloadItem.prototype = {
         clipboard.copyString(this.uri, document);
         break;
     }
   },
 };
 
 let gDownloadList = {
   downloadItemsMap: new Map(),
-  downloadItems: {},
+  idToDownloadItemMap: new Map(),
   _autoIncrementID: 0,
   downloadView: null,
   searchBox: null,
   searchTerms: [],
   searchAttributes: [ "target", "status", "dateTime", ],
   contextMenus: [
     // DOWNLOAD_DOWNLOADING
     [
@@ -722,17 +722,17 @@ let gDownloadList = {
       return;
     }
 
     // Clear the list before adding items by replacing with a shallow copy.
     let empty = this.downloadView.cloneNode(false);
     this.downloadView.parentNode.replaceChild(empty, this.downloadView);
     this.downloadView = empty;
 
-    for each (let downloadItem in this.downloadItems) {
+    for (let downloadItem of this.idToDownloadItemMap.values()) {
       if (downloadItem.inProgress ||
           downloadItem.matchesSearch(this.searchTerms, this.searchAttributes)) {
         this.downloadView.appendChild(downloadItem.element);
 
         // Because of the joys of XBL, we can't update the buttons until the
         // download object is in the document.
         downloadItem.updateView();
       }
@@ -768,17 +768,17 @@ let gDownloadList = {
   /**
    * Update the disabled state of the clear list button based on whether or not
    * there are items in the list that can potentially be removed.
    */
   updateClearListButton: function() {
     let button = document.getElementById("clearListButton");
 
     // The button is enabled if we have items in the list that we can clean up.
-    for each (let downloadItem in this.downloadItems) {
+    for (let downloadItem of this.idToDownloadItemMap.values()) {
       if (!downloadItem.inProgress &&
           downloadItem.matchesSearch(this.searchTerms, this.searchAttributes)) {
         button.disabled = false;
         return;
       }
     }
 
     button.disabled = true;
@@ -803,18 +803,17 @@ let gDownloadList = {
     }
 
     let popup = document.getElementById("downloadContextMenu");
     while (popup.hasChildNodes()) {
       popup.removeChild(popup.firstChild);
     }
 
     if (this.downloadView.selectedItem) {
-      let dl = this.downloadView.selectedItem;
-      let downloadItem = this.downloadItems[dl.getAttribute("id")];
+      let downloadItem = this._getSelectedDownloadItem();
 
       let idx = downloadItem.state;
       if (idx < 0) {
         idx = 0;
       }
 
       let menus = this.contextMenus[idx];
       for (let i = 0; i < menus.length; ++i) {
@@ -833,23 +832,21 @@ let gDownloadList = {
     return false;
   },
 
   /**
    * Perform the default action for the currently selected download item.
    */
   doDefaultForSelected: function() {
     // Make sure we have something selected.
-    let item = this.downloadView.selectedItem;
-    if (!item) {
+    let download = this._getSelectedDownloadItem();
+    if (!download) {
       return;
     }
 
-    let download = this.downloadItems[item.getAttribute("id")];
-
     // Get the default action (first item in the menu).
     let menuitem = document.getElementById(this.contextMenus[download.state][0]);
 
     // Try to do the action if the command is enabled.
     download.doCommand(menuitem.getAttribute("cmd"));
   },
 
   onDownloadDblClick: function(aEvent) {
@@ -886,27 +883,27 @@ let gDownloadList = {
 
     let elm = aItem;
 
     while (elm.nodeName != "richlistitem" ||
            elm.getAttribute("type") != "download") {
       elm = elm.parentNode;
     }
 
-    let downloadItem = this.downloadItems[elm.getAttribute("id")];
+    let downloadItem = this._getDownloadItemForElement(elm);
     downloadItem.doCommand(aCmd);
   },
 
   onDragStart: function(aEvent) {
-    if (!this.downloadView.selectedItem) {
+    let downloadItem = this._getSelectedDownloadItem();
+    if (!downloadItem) {
       return;
     }
 
     let dl = this.downloadView.selectedItem;
-    let downloadItem = this.downloadItems[dl.getAttribute("id")];
     let f = downloadItem.localFile;
     if (!f.exists()) {
       return;
     }
 
     let dt = aEvent.dataTransfer;
     dt.mozSetDataAt("application/x-moz-file", f, 0);
     let url = Services.io.newFileURI(f).spec;
@@ -978,17 +975,17 @@ let gDownloadList = {
     this.downloadView.selectAll();
   },
 
   onUpdateProgress: function() {
     let numActive = 0;
     let totalSize = 0;
     let totalTransferred = 0;
 
-    for each (let downloadItem in this.downloadItems) {
+    for (let downloadItem of this.idToDownloadItemMap.values()) {
       if (!downloadItem.inProgress) {
         continue;
       }
 
       numActive++;
 
       // Only add to total values if we actually know the download size.
       if (downloadItem.maxBytes > 0 &&
@@ -1025,17 +1022,17 @@ let gDownloadList = {
     document.title = title;
   },
 
   onDownloadAdded: function(aDownload) {
     let newID = this._autoIncrementID++;
 
     let downloadItem = new DownloadItem(newID, aDownload);
     this.downloadItemsMap.set(aDownload, downloadItem);
-    this.downloadItems[newID] = downloadItem;
+    this.idToDownloadItemMap.set(newID, downloadItem);
 
     if (downloadItem.inProgress ||
         downloadItem.matchesSearch(this.searchTerms, this.searchAttributes)) {
       this.downloadView.appendChild(downloadItem.element);
 
       // Because of the joys of XBL, we can't update the buttons until the
       // download object is in the document.
       downloadItem.updateView();
@@ -1083,17 +1080,17 @@ let gDownloadList = {
   onDownloadRemoved: function(aDownload) {
     let downloadItem = this.downloadItemsMap.get(aDownload);
     if (!downloadItem) {
       Cu.reportError("Download doesn't exist.");
       return;
     }
 
     this.downloadItemsMap.delete(aDownload);
-    delete this.downloadItems[downloadItem.id];
+    this.idToDownloadItemMap.delete(downloadItem.id);
 
     this.removeFromView(downloadItem);
   },
 
   removeFromView: function(aDownloadItem) {
     // Make sure we have an item to remove.
     if (!aDownloadItem.element) {
       return;
@@ -1101,16 +1098,23 @@ let gDownloadList = {
 
     let index = this.downloadView.selectedIndex;
     this.downloadView.removeChild(aDownloadItem.element);
     this.downloadView.selectedIndex = Math.min(index, this.downloadView.itemCount - 1);
 
     // We might have removed the last item, so update the clear list button.
     this.updateClearListButton();
   },
+  _getDownloadItemForElement(element) {
+    return this.idToDownloadItemMap.get(element.getAttribute("id"));
+  },
+  _getSelectedDownloadItem() {
+    let dl = this.downloadView.selectedItem;
+    return dl ? this._getDownloadItemForElement(dl) : null;
+  },
 };
 
 function Startup() {
   // Convert strings to those in the string bundle.
   let sb = document.getElementById("downloadStrings");
   let strings = ["paused", "cannotPause", "doneStatus", "doneSize",
                  "doneSizeUnknown", "stateFailed", "stateCanceled",
                  "stateBlocked", "stateBlockedPolicy", "stateDirty",