The images.... in the css there was a display:block - I've put it back in there so you can see it. Do you know why it looks fine in IE, Chrome and Safari but is doing that in FF?
This is how it was in the unedited version of the theme:
img.attachment-homepage-thumbs {margin: 0 auto 5px auto; display:block;}
so I just duplicated for those thumbnails/featured images for the home page.
When I took that line out (display:block;) it looks fine.