In FF3, a disabled window will still close on esc unless the user first attempts to focus the disabled window.

Execute the code below, and once the page loads, pressing esc will close the window in FF3, but not in IE6. (Didn't test in other browsers). If you click the window first, esc will function as expected.

Code:
Ext.onReady(function() {
/* Hack-fix
  Ext.override(Ext.Window,{
    onEsc:function(){
      if(!this.disabled){
        this[this.closeAction]();
      }
    }
  });
*/

  var win = new Ext.Window({
    title:'my_window',
    html:'body',
    width:500,
    height:500
  });
  
  win.show();
  win.disable();

});
Included is a hack that seems to solve it.