I've been trying to get my javascript expand-boxes to work the way I want them to in Firefox, but it just won't work

I mean boxes which expand when you press a lil button like "Additional Options" or "Posting Rules" here on this vbulletin board. I even copied this code to try but it was the same problem for me

The problem is.. whenever I expand the box, and then un-expand it (or just un-expand it) a white space is left at the bottom of the page with the same size as the content of the box. This only happens in firefox; opera and IE works fine.

I think it has something to do with tables, because this doesn't happen when I move the code for the expanding box outside a table. Even if I just put it in a simple table like this it happens;

<table width="500" border="0" cellspacing="0" cellpadding="0">


I think it's because of the tables cuz it works at this board, but it's not so heavy on tables I think. Anyone have any ideas? Sorry if it's the wrong part of the message board!