Iframe or Div width with a planning control (with Firefox 3.0)

Hello,

When I refresh my Iframe (same problem with Div) , I see planning control 'shrink' under Firefox 3.0 (beta 4) but with IE7 the width is always the same.
After Iframe is refresh, planning width is back to normal.
If i put something else (button, iframe, whatever) width is always 100% as I want, but not with Planning control.
And no problem when I set a width in pixels but I need to have something dynamic.

Note : I leave Layout/Width is empty = width 100% , I think there is a problem there

What can I do ?

Thanks