Well, as I've mentioned in the past, as long as we can have identical pixel heights across all browsers, operating systems and devices, we can use whatever font definitions at all, it doesn't matter to me.  But if a given font definition practice makes Firefox have 18px high fonts in a given UI widget, and 19px in Chromium, and that makes, say, a line break in different places in different browsers, that seems highly undesirable to me.