diff --git a/xul/content/commands.js b/xul/content/commands.js
--- a/xul/content/commands.js
+++ b/xul/content/commands.js
@@ -3747,7 +3747,7 @@
     {
         // open Mozilla/SeaMonkey preferences
         const PREF_URL = 'chrome://chatzilla/content/pref-irc.xul';
-        if (goPreferences.arity == 1) // SeaMonkey 2.x
+        if (goPreferences.length == 1) // SeaMonkey 2.x
             goPreferences('navigator_pane');
         else // Mozilla, SeaMonkey 1.x, etc.
             goPreferences('navigator', PREF_URL, 'navigator');