diff --git a/widget/gtk/nsWindow.cpp b/widget/gtk/nsWindow.cpp
--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -3242,6 +3242,12 @@
   mBounds = aRect;
   ConstrainSize(&mBounds.width, &mBounds.height);
 
+  // eWindowType_child is not supported on Wayland. Just switch to toplevel
+  // as a workaround.
+  if (!mIsX11Display && mWindowType == eWindowType_child) {
+    mWindowType = eWindowType_toplevel;
+  }
+
   // figure out our parent window
   GtkWidget *parentMozContainer = nullptr;
   GtkContainer *parentGtkContainer = nullptr;