Something to bear in mind is that IE7 uses an algorithm called cleartype to display HTML pages; this is on within IE7 by default, but I don't understand why Firefox would display relatively poorer fonts. It is a finishing effect on fonts to enhance them on flat screen and TFT monitors. Ironically it looks pants on a CRT!
Cleartype is also available as a global option in Windows - perhaps something to try...
Control Panel>Display>Appearance>Effects and choose 'cleartype' from the dropdown.
Rgds
James