--- a/testing/marionette/browser.js
+++ b/testing/marionette/browser.js
@@ -154,17 +154,16 @@ browser.Context = class {
     // We should have one frame.Manager per browser.Context so that we
     // can handle modals in each <xul:browser>.
     this.frameManager = new frame.Manager(driver);
     this.frameRegsPending = 0;
 
     // register all message listeners
     this.frameManager.addMessageManagerListeners(driver.mm);
     this.getIdForBrowser = driver.getIdForBrowser.bind(driver);
-    this.updateIdForBrowser = driver.updateIdForBrowser.bind(driver);
   }
 
   /**
    * Returns the content browser for the currently selected tab.
    * If there is no tab selected, null will be returned.
    */
   get contentBrowser() {
     if (this.tab) {
@@ -389,17 +388,16 @@ browser.Context = class {
     if (this.tabBrowser) {
       // If we're setting up a new session on Firefox, we only process the
       // registration for this frame if it belongs to the current tab.
       if (!this.tab) {
         this.switchToTab();
       }
 
       if (target === this.contentBrowser) {
-        this.updateIdForBrowser(this.contentBrowser, uid);
         this._needsFlushPendingCommands = true;
       }
     }
 
     // used to delete sessions
     this.knownFrames.push(uid);
   }
 

--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -136,17 +136,16 @@ this.GeckoDriver = function(appId, serve
   this.mainFrame = null;
   // chrome iframe that currently has focus
   this.curFrame = null;
   this.mozBrowserClose = null;
   this.currentFrameElement = null;
   // frame ID of the current remote frame, used for mozbrowserclose events
   this.oopFrameId = null;
   this.observing = null;
-  this._browserIds = new WeakMap();
 
   // The curent context decides if commands should affect chrome- or
   // content space.
   this.context = Context.Content;
 
   this.sandboxes = new Sandboxes(() => this.getCurrentWindow());
   this.legacyactions = new legacyaction.Chain();
 
@@ -1321,43 +1320,21 @@ GeckoDriver.prototype.refresh = async fu
     this.mm.broadcastAsyncMessage(
         "Marionette:waitForPageLoaded" + this.curBrowser.curFrameId,
         parameters);
   });
 
   await refresh;
 };
 
-/**
- * Forces an update for the given browser's id.
- */
-GeckoDriver.prototype.updateIdForBrowser = function(browser, newId) {
-  this._browserIds.set(browser.permanentKey, newId);
-};
-
-/**
- * Retrieves a listener id for the given xul browser element. In case
- * the browser is not known, an attempt is made to retrieve the id from
- * a CPOW, and null is returned if this fails.
- */
 GeckoDriver.prototype.getIdForBrowser = function(browser) {
   if (browser === null) {
     return null;
   }
-  let permKey = browser.permanentKey;
-  if (this._browserIds.has(permKey)) {
-    return this._browserIds.get(permKey);
-  }
-
-  let winId = browser.outerWindowID;
-  if (winId) {
-    this._browserIds.set(permKey, winId);
-    return winId;
-  }
-  return null;
+  return browser.outerWindowID;
 },
 
 /**
  * Get the current window's handle. On desktop this typically corresponds
  * to the currently selected tab.
  *
  * Return an opaque server-assigned identifier to this window that
  * uniquely identifies it within this Marionette instance.  This can