Removing vertical scrollbar from selectbox?

Hi,

I've got a selectbox with the 'multiple' property - so it's not a
dropdown list but a multi-select thingy. When items are added or removed
the selectbox automatically changes its size so everything fits in. This
all works fine...

But in firefox the vertical scrollbar just keeps being displayed! Even
when I resize the selectbox size to '40' with only 5 items in it, there
still is a scrollbar :S.

Is there any way to remove this scrollbar? Please notice this is a
problem in firefox - IE shows the selectbox correctly (only a scrollbar
when needed).

I tried all kind of thing, like 'selectbox.scrolling = no',
'selectbox.scrolling = auto',
'selectbox.style.vertical-scrollbar=hide'... nothing seems to work :'(

Any help is highly appriciated

Harrie