They very well could be. they both render the site differently and they both use different fonts.
IE uses the Fonts that Windows uses. Firefox has its own font set cause it is used on more than just Windows. So it comes with a font set that can be used on Widnows, OS X and Linux.
IE uses the Fonts that Windows uses. Firefox has its own font set cause it is used on more than just Windows. So it comes with a font set that can be used on Widnows, OS X and Linux.