Text boxes and such vary by font and character size - they are sized for x characters/columns wide by y lines tall. The gecko browsers, at least at larger screen resolutions, tend to display text smaller than ie does. So, a box that's 50 characters wide looks smaller in gecko than in ie.
On my 1600x1200 display, I have to increase font's by 120-125% in mozilla/firefox to get them close to ie visual representations.
One solution might be to use css to define font sizes by % - e.g. normal = 100% and change the css to increase the size by 20-25% if gecko's are detected. - Note that some font's scale better than others, and some scale well in certain sizes, put poorly in other sizes. You'll have to do a little experimentation.