diff --git a/src/client/connection.ts b/src/client/connection.ts
index bf0461d640..cc0e01dc64 100644
--- a/src/client/connection.ts
+++ b/src/client/connection.ts
@@ -122,7 +122,7 @@ export class Connection {
     }
     const object = this._objects.get(guid);
     if (!object)
-      throw new Error(`Cannot find object to call "${method}": ${guid}`);
+      throw new Error(`Cannot find object to emit "${method}": ${guid}`);
     object._channel.emit(method, this._replaceGuidsWithChannels(params));
   }
 
diff --git a/src/server/frames.ts b/src/server/frames.ts
index 043df215e6..8ce888a285 100644
--- a/src/server/frames.ts
+++ b/src/server/frames.ts
@@ -1056,7 +1056,10 @@ export class Frame extends EventEmitter {
 
   _startNetworkIdleTimer() {
     assert(!this._networkIdleTimer);
-    if (this._firedLifecycleEvents.has('networkidle'))
+    // We should not start a timer and report networkidle in detached frames.
+    // This happens at least in Firefox for child frames, where we may get requestFinished
+    // after the frame was detached - probably a race in the Firefox itself.
+    if (this._firedLifecycleEvents.has('networkidle') || this._detached)
       return;
     this._networkIdleTimer = setTimeout(() => this._onLifecycleEvent('networkidle'), 500);
   }