Trying to dictate fonts is always an issue because you cannot guarantee that the visitor will have the fonts that you specify. Remember that once you start playing with this area its not just a matter of IE versus Firefox any more, you need to check that whatever you do doesn't mess up any of the various phone browsers.
Best thing is to leave your page design fluid and let the user use their own browser defaults.
If you want to make a page look the same in your two browsers then change the browser properties to be the same.
Hi Peter, it could be a Firefox issue. IE7 is almost giving me the correct colour and Google Chrome seems to be perfect. Bravenet Support reckon that the latest version of Firefox can be problematic.