diff -urN firefox-152.0.3-orig/widget/gtk/nsFilePicker.cpp firefox-152.0.3/widget/gtk/nsFilePicker.cpp
--- firefox-152.0.3-orig/widget/gtk/nsFilePicker.cpp	2026-06-25 10:03:14 +0900
+++ firefox-152.0.3/widget/gtk/nsFilePicker.cpp	2026-07-03 04:34:57 +0900
@@ -52,14 +52,6 @@
 
 static nsIFile* sPrevDisplayDirectory = nullptr;
 
-// Use an application-defined response ID (non-negative) for the accept button
-// instead of GTK_RESPONSE_ACCEPT (-3). GTK's built-in negative response IDs
-// cause the button to be treated as the dialog's default widget, meaning it
-// activates on Enter. A non-negative ID prevents this, so a page that tricks
-// the user into holding Enter before the dialog appears cannot auto-confirm
-// an unintended file upload. See bug 2033848 and the equivalent Chrome fix.
-static const gint kFilePickerAccept = 0;
-
 void nsFilePicker::Shutdown() { NS_IF_RELEASE(sPrevDisplayDirectory); }
 
 #ifdef MOZ_ENABLE_DBUS
@@ -698,7 +690,7 @@
 
   GtkFileChooser* file_chooser = GTK_FILE_CHOOSER(gtk_file_chooser_dialog_new(
       title.get(), parent_widget, action, g_dgettext("gtk30", "_Cancel"),
-      GTK_RESPONSE_CANCEL, accept_button, kFilePickerAccept, nullptr));
+      GTK_RESPONSE_CANCEL, accept_button, GTK_RESPONSE_ACCEPT, nullptr));
 
   // If we have --enable-proxy-bypass-protection, then don't allow
   // remote URLs to be used.
@@ -774,6 +766,11 @@
     }
   }
 
+  if (GTK_IS_DIALOG(file_chooser)) {
+    gtk_dialog_set_default_response(GTK_DIALOG(file_chooser),
+                                    GTK_RESPONSE_ACCEPT);
+  }
+
   size_t count = mFilters.Length();
   for (size_t i = 0; i < count; ++i) {
     GtkFileFilter* filter = NewFilter(mFilters[i], mFilterNames[i]);
@@ -878,8 +875,7 @@
   nsIFilePicker::ResultCode result;
   switch (response) {
     case GTK_RESPONSE_OK:
-    case GTK_RESPONSE_ACCEPT:  // emitted by GTK internally on double-click
-    case kFilePickerAccept:
+    case GTK_RESPONSE_ACCEPT:
       ReadValuesFromNonPortalFileChooser(GTK_FILE_CHOOSER(file_chooser));
       result = nsIFilePicker::returnOK;
       break;