I have realized the problem before but I guess it didn\'t matter as much then as it did now.
What I discovered is that Firefox has a default line-height value of 1.2
As far as I know from experience the line-height on input won't change unless you change the font-size - than the line-height will change to be the same as font-size + 4px (2 top 2 bot i guess).
In case this bothers you with styling, you can use the top and bottom padding to achieve a fake bigger line-height effect.