In Win7 (checked on latest nightly for 2010-12-14), Firefox renders Aero-themed input box below until mouseover, when red background style gets applied. This changes back to old rendering mode, but a bug makes the input box become 2px narrower (in conjunction with adding a label element after the input) and so the text to the right moves left 2px.
However, when the old rendering mode is triggered from the get-go, by always applying a background color to the input element (white until mouseover), the width doesn't change. The bug doesn't happen for me in IE, Opera, or Safari.