When using Firefox, there seems to be extra spacing added around the text value displayed. This isn't just vertical space caused by line-height adjustments, but horizontal space as well.
In contrast, Chrome, Opera (which has a slightly different issue with line-height), and even IE all display submit buttons without adding any extra space.
You can view an example of this behavior here: http://jsfiddle.net/jswartwood/aFCwj/
If you inspect the elements using Firebug and hover over the <a> and <input> tags, you'll notice that it's not due to padding or other styling properties.
According to the bug tracker, it appears that Firefox places a "block" inside these form elements. This raises the question - why? It makes maintaining consistent visual button sizes very challenging.