diff --git a/browser_patches/firefox/BUILD_NUMBER b/browser_patches/firefox/BUILD_NUMBER
index 7986d28c50..0a468d3fd9 100644
--- a/browser_patches/firefox/BUILD_NUMBER
+++ b/browser_patches/firefox/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1172
-Changed: yurys@chromium.org Wed Sep  9 23:06:13 PDT 2020
+1173
+Changed: yurys@chromium.org Thu Sep 10 12:35:25 PDT 2020
diff --git a/browser_patches/firefox/juggler/TargetRegistry.js b/browser_patches/firefox/juggler/TargetRegistry.js
index d0b06334d9..b205d6da2a 100644
--- a/browser_patches/firefox/juggler/TargetRegistry.js
+++ b/browser_patches/firefox/juggler/TargetRegistry.js
@@ -196,11 +196,6 @@ class TargetRegistry {
       const browserContext = this._userContextIdToBrowserContext.get(userContextId);
       if (browserContext && browserContext.defaultViewportSize)
         setViewportSizeForBrowser(browserContext.defaultViewportSize, tab.linkedBrowser, window);
-
-      const linkedBrowser = tab.linkedBrowser;
-      const target = this._browserToTarget.get(linkedBrowser);
-      if (target)
-          target._pageWindowReadyCallback();
     };
 
     const onTabCloseListener = event => {
@@ -325,7 +320,6 @@ class TargetRegistry {
     });
     if (browserContext && browserContext.defaultViewportSize)
       setViewportSizeForBrowser(browserContext.defaultViewportSize, browser, window);
-    target._pageWindowReadyCallback();
     browser.focus();
     if (browserContext.settings.timezoneId) {
       if (await target.hasFailedToOverrideTimezone())
@@ -382,7 +376,10 @@ class PageTarget {
     browserContext._firstPageCallback();
     this._registry._browserToTarget.set(this._linkedBrowser, this);
     this._registry._browserBrowsingContextToTarget.set(this._linkedBrowser.browsingContext, this);
-    this._pageWindowReady = new Promise(r => this._pageWindowReadyCallback = r);
+  }
+
+  async windowReady() {
+    await waitForWindowReady(this._window);
   }
 
   linkedBrowser() {
diff --git a/browser_patches/firefox/juggler/protocol/PageHandler.js b/browser_patches/firefox/juggler/protocol/PageHandler.js
index 816ffc2630..e5222d40a1 100644
--- a/browser_patches/firefox/juggler/protocol/PageHandler.js
+++ b/browser_patches/firefox/juggler/protocol/PageHandler.js
@@ -140,7 +140,7 @@ class PageHandler {
       // On Mac the window may not yet be visible when TargetCreated and its
       // NSWindow.windowNumber may be -1, so we wait until the window is known
       // to be initialized and visible.
-      await this._pageTarget._pageWindowReady;
+      await this._pageTarget.windowReady();
       await this.startVideoRecording(Object.assign({file}, options));
     }
   }