diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index d51172f429..0a92c6d17f 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -1145,6 +1145,7 @@ pref("dom.disable_window_open_feature.menubar",     false);
 pref("dom.disable_window_open_feature.resizable",   true);
 pref("dom.disable_window_open_feature.minimizable", false);
 pref("dom.disable_window_open_feature.status",      true);
+pref("dom.disable_window_showModalDialog",          true);
 
 pref("dom.allow_scripts_to_close_windows",          false);