diff --git a/browser_patches/firefox/BUILD_NUMBER b/browser_patches/firefox/BUILD_NUMBER
index 425c0e77e0..7986d28c50 100644
--- a/browser_patches/firefox/BUILD_NUMBER
+++ b/browser_patches/firefox/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1171
-Changed: aslushnikov@gmail.com Wed Sep  9 09:33:56 PDT 2020
+1172
+Changed: yurys@chromium.org Wed Sep  9 23:06:13 PDT 2020
diff --git a/browser_patches/firefox/juggler/TargetRegistry.js b/browser_patches/firefox/juggler/TargetRegistry.js
index 620da981c6..d0b06334d9 100644
--- a/browser_patches/firefox/juggler/TargetRegistry.js
+++ b/browser_patches/firefox/juggler/TargetRegistry.js
@@ -196,6 +196,11 @@ 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 => {
@@ -320,6 +325,7 @@ class TargetRegistry {
     });
     if (browserContext && browserContext.defaultViewportSize)
       setViewportSizeForBrowser(browserContext.defaultViewportSize, browser, window);
+    target._pageWindowReadyCallback();
     browser.focus();
     if (browserContext.settings.timezoneId) {
       if (await target.hasFailedToOverrideTimezone())
@@ -376,6 +382,7 @@ 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);
   }
 
   linkedBrowser() {
diff --git a/browser_patches/firefox/juggler/protocol/PageHandler.js b/browser_patches/firefox/juggler/protocol/PageHandler.js
index 78d0a057b9..816ffc2630 100644
--- a/browser_patches/firefox/juggler/protocol/PageHandler.js
+++ b/browser_patches/firefox/juggler/protocol/PageHandler.js
@@ -137,6 +137,10 @@ class PageHandler {
     const options = this._pageTarget.browserContext().screencastOptions;
     if (options) {
       const file = OS.Path.join(options.dir, helper.generateId() + '.webm');
+      // 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.startVideoRecording(Object.assign({file}, options));
     }
   }