It's always good to do font sizes in em's rather than px or pt. If you use px or pt, IE won't resize the font if people need to see larger (or smaller) text. Your default font size (set in the body tag of the CSS file) should always be set at 100%. Then you can change the size in your p tags (or whatever) as you see fit. Normally, I use 82%. Then the fonts can be resized at the end user's discretion.
I'm not sure what you mean about "do what you want in terms of colors" - but in Forefox, to change your base font size, you just need to hold down the "CTRL" button, and then hit the "+" or "-" key. "CTRL+0" resets it to the default size set by the stylesheet.
Moshu is correct though - IE and Firefox generally show font sizes the same way...so if FF is huge for you, it must be your browser setting.
Don't forget, people also accidentally change their font size all the time without being aware they've done so until they font shows up really huge or really tiny. I get clients that complain about this all the time, until I show them that they've accidentally messed with their browser settings. Happens *a lot* in IE - where if you have a scroll wheel in your mouse, all you have to to do right-click and scroll, and the font size changes.