Drop below letters like g, y, j are truncated in Firefox 3.6 only.
<input type="text" value="yyyy gggg xxxx" style="height: 1em;" />

http://jsfiddle.net/mrtsherman/yqTjX/
My google-fu is completely failing me. Simple problem. I imagine there is a simple solution.
1em represents the width of a lowercase M in the given font, so I understand. 1em won’t necessarily produce enough height for a full character to be displayed. Because of differences in font rendering between browsers (especially Firefox and Safari), you may notice subtle differences along these lines. Either remove the height (so it’s automatically calculated), or increase the size. Nice and simple.