Hi,
I think it's marked as fixed... already in 3.0 version tree, so not (and probably never ) for 2.0
Anyway I'm not sure because I'm not a firefox expert.
I like firefox (I'm using it this moment), but it has bugs as other browsers, one of my favorite: firefox can not render 1 row height textarea, very stupid. This is not fixed from many years... 
Back to the workaround:
I'm using this usually, just copied from one of my aspx page:
<div style="overflow: auto;"><input type="text" id="txt1" value="something" /></div><%-- The surrounding div is not necessary, but it eliminates missing cursor bug in Firefox. --%>
Unfortunately this not works always, implementing for a grid view is not easy.
I recommend to play with surrounding divs (around your grid) and the overflow css property (try all setting: scroll, visible, hidden, auto) OR try to remove overflow css from anywhere. The firefox bug page also have some workaround in the attachments section. Maybe these helps.