--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -196,39 +196,32 @@ Object.defineProperty(GeckoDriver.protot
       if (tabBrowser) {
         tabBrowser.tabs.forEach(tab => {
           let winId = this.getIdForBrowser(browser.getBrowserForTab(tab));
           if (winId !== null) {
             hs.push(winId);
           }
         });
       } else {
-        // For other chrome windows beside the browser window, only count the window itself.
-        let winId = win.QueryInterface(Ci.nsIInterfaceRequestor)
-            .getInterface(Ci.nsIDOMWindowUtils)
-            .outerWindowID;
-        hs.push(winId.toString());
+        // For other chrome windows beside the browser window, only add the window itself.
+        hs.push(getOuterWindowId(win));
       }
     }
 
     return hs;
   },
 });
 
 Object.defineProperty(GeckoDriver.prototype, "chromeWindowHandles", {
   get : function () {
     let hs = [];
     let winEn = Services.wm.getEnumerator(null);
 
     while (winEn.hasMoreElements()) {
-      let foundWin = winEn.getNext();
-      let winId = foundWin.QueryInterface(Ci.nsIInterfaceRequestor)
-          .getInterface(Ci.nsIDOMWindowUtils)
-          .outerWindowID;
-      hs.push(winId.toString());
+      hs.push(getOuterWindowId(winEn.getNext()));
     }
 
     return hs;
   },
 });
 
 GeckoDriver.prototype.QueryInterface = XPCOMUtils.generateQI([
   Ci.nsIMessageListener,
@@ -354,19 +347,18 @@ GeckoDriver.prototype.addFrameCloseListe
  * @param {nsIDOMWindow} win
  *     Window for which we will create a browsing context.
  *
  * @return {string}
  *     Returns the unique server-assigned ID of the window.
  */
 GeckoDriver.prototype.addBrowser = function (win) {
   let bc = new browser.Context(win, this);
-  let winId = win.QueryInterface(Ci.nsIInterfaceRequestor)
-      .getInterface(Ci.nsIDOMWindowUtils).outerWindowID;
-  winId = winId + ((this.appName == "B2G") ? "-b2g" : "");
+  let winId = getOuterWindowId(win);
+
   this.browsers[winId] = bc;
   this.curBrowser = this.browsers[winId];
   if (!this.wins.has(winId)) {
     // add this to seenItems so we can guarantee
     // the user will get winId as this window's id
     this.wins.set(winId, win);
   }
 };
@@ -1199,23 +1191,16 @@ GeckoDriver.prototype.setWindowPosition 
  *
  * @param {string} name
  *     Target name or ID of the window to switch to.
  */
 GeckoDriver.prototype.switchToWindow = function* (cmd, resp) {
   let switchTo = cmd.parameters.name;
   let found;
 
-  let getOuterWindowId = function (win) {
-    let rv = win.QueryInterface(Ci.nsIInterfaceRequestor)
-        .getInterface(Ci.nsIDOMWindowUtils)
-        .outerWindowID;
-    return rv;
-  };
-
   let byNameOrId = function (name, outerId, contentWindowId) {
     return switchTo == name ||
         switchTo == contentWindowId ||
         switchTo == outerId;
   };
 
   let winEn = Services.wm.getEnumerator(null);
   while (winEn.hasMoreElements()) {
@@ -2851,8 +2836,25 @@ GeckoDriver.prototype.commands = {
 function copy (obj) {
   if (Array.isArray(obj)) {
     return obj.slice();
   } else if (typeof obj == "object") {
     return Object.assign({}, obj);
   }
   return obj;
 }
+
+/**
+ * Get the outer window ID for the specified window.
+ *
+ * @param {nsIDOMWindow} win
+ *     Window whose browser we need to access.
+ *
+ * @return {string}
+ *     Returns the unique window ID.
+ */
+function getOuterWindowId(win) {
+  let id = win.QueryInterface(Ci.nsIInterfaceRequestor)
+      .getInterface(Ci.nsIDOMWindowUtils)
+      .outerWindowID;
+
+  return id.toString();
+}