The reason the text appears more blockier in Firefox is because there is no 'anti-aliasing' for the text itself. Microsoft implements their 'ClearType' font which smooths out the edges in the text. I'm sure the same setting is available in Firefox, if not by selecting another font and size by trying that out.
Also, if it appears too big, press Ctrl + 0 to reset the zoom level, or zoom out slightly to make it smaller... Firefox is a great browser, although I am using Opera now for the time being because its 'awesome bar' isn't so awesome...