diff --git a/browser_patches/firefox/BUILD_NUMBER b/browser_patches/firefox/BUILD_NUMBER
index 01d8949b80..358b8dbc3a 100644
--- a/browser_patches/firefox/BUILD_NUMBER
+++ b/browser_patches/firefox/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1246
-Changed: lushnikov@chromium.org Tue 20 Apr 2021 10:26:28 AM PDT
+1247
+Changed: dgozman@gmail.com Fri Apr 23 15:46:40 PDT 2021
diff --git a/browser_patches/firefox/juggler/protocol/BrowserHandler.js b/browser_patches/firefox/juggler/protocol/BrowserHandler.js
index a084b3389c..92dea19214 100644
--- a/browser_patches/firefox/juggler/protocol/BrowserHandler.js
+++ b/browser_patches/firefox/juggler/protocol/BrowserHandler.js
@@ -138,7 +138,12 @@ class BrowserHandler {
       "navigator:browser"
     );
     if (browserWindow && browserWindow.gBrowserInit) {
-      await browserWindow.gBrowserInit.idleTasksFinishedPromise;
+      // idleTasksFinishedPromise does not resolve when the window
+      // is closed early enough, so we race against window closure.
+      await Promise.race([
+        browserWindow.gBrowserInit.idleTasksFinishedPromise,
+        waitForWindowClosed(browserWindow),
+      ]);
     }
     // Try to fully initialize browser before closing.
     // See comment in `Browser.enable`.
@@ -281,6 +286,22 @@ async function waitForAddonManager() {
   });
 }
 
+async function waitForWindowClosed(browserWindow) {
+  if (browserWindow.closed)
+    return;
+  await new Promise((resolve => {
+    const listener = {
+      onCloseWindow: window => {
+        if (window === browserWindow) {
+          Services.wm.removeListener(listener);
+          resolve();
+        }
+      },
+    };
+    Services.wm.addListener(listener);
+  }));
+}
+
 function nullToUndefined(value) {
   return value === null ? undefined : value;
 }