--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -1213,16 +1213,17 @@ pref("dom.disable_window_open_feature.ti
 pref("dom.disable_window_open_feature.close",       false);
 pref("dom.disable_window_open_feature.toolbar",     false);
 pref("dom.disable_window_open_feature.location",    false);
 pref("dom.disable_window_open_feature.personalbar", false);
 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);
 
 pref("dom.require_user_interaction_for_beforeunload", true);
 
 pref("dom.disable_open_during_load",                false);
 pref("dom.popup_maximum",                           20);
 pref("dom.popup_allowed_events", "change click dblclick mouseup pointerup notificationclick reset submit touchend");