Is there a way to get this working in FF?
No, and it doesn't work in most common versions of IE, either. IE will open the dialog, but once you've selected a file with it the form won't actually submit.
Abandon hope. The only way to fake a file upload box is using the transparency technique, and that's really not recommended at all as browsers may lay out file upload boxes differently internally (or even provide a file upload control that doesn't include a Browse dialogue), making it highly likely you'll end up with an inoperable form.
Learn to love the grey file upload field, or use progressive enhancement to replace it with Flash where available.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
