yes, I know that this thread is old, but i stumbeled across it via google, having the same problem, and I decided to share my solution with you (in case anyone else is searching for this):
The problem here is, that the Ext.KeyNav from the menu is catching the tabulator key.
To solve this, I simply destroyed it when showing the menu - I only display a form in the menu and don't need a menu navigation for it:
this.mnu = new Ext.menu.Menu (
show: function (mnu)