Since FF 3.0.x the fontsize in select boxes is to big.
In prosilver standard fontsize for selects is definded here:
But this doesn't work in FF 3.0.x
The css for fieldset remains.
you change the definition for select:
Adding font-size to select will fix the bug. The change has no effects on Opera 9, Internet Explorer 7 and Internet Explorer 6, Firefox 2.