Jumping fixed header after closing dialog (firefox)

Jumping fixed header after closing dialog (firefox)

i have a problem, you can see that here (with firefox) :  http://fiddle.jshell.net/ckuskun/6rZxG/2/show/light/
when click to " open" link, a dialog is opening. after closing this dialog, fixed header is jumping to the top.
how can i fix this problem