The reason they look too small in Firefox was because apparently your font size was larger in IE, at least it looks that way.
BTW: You might want to enable ClearType font smoothing to get the same font smoothing in Firefox as there is in IE. You can do this by right clicking your desktop, click Properties, click Appearance, then Effects, and change the "Use the following method to smooth edges of screen fonts" setting to ClearType. Or if you prefer, you can adjust it via
http://www.microsoft.com/typography/cleartype/tuner/Step1.aspx (open in IE) which will allow you to fine tune ClearType settings.