I used firebug but it didnt help much with this issue, so what I did was comment out ALL of the form formatting in my themes style.css by putting a /* and a */ before and after all of it, so basically all my theme form formatting was removed... and it fixed it. my form looked bad but it showed me that the problem came from a conflict in my themes css vs. the input field styles in the woocommerce.css.
try it, then one by one add your styles back and see where it causes the problem