From looking at it, it looks like a "float" issue on one of your divs. I would wrap the entire form in a div but make sure if you want to place t beside that other div, that it needs to 1). also be floated (you can float left or right and then determine the gutter space between the two using margin) and 2.) have the correct width set. If the width of the div is wider than the space allotted - OR - if you don't set a width on the floated div, I believe Firefox makes the assumption that the div is 100% wide if it is specified as a "block" element.
You may also have something clearing the float prematurely.