Fixed Text Size

I'm sure I knew how to do this before but I've kinda forgotten... how do you make the text a fixed size (so that changing the text size in a browser doesn't affect it)?

EDIT - Geh sorry, remembered to use pixel size...

EDIT Again - seems to work fine in IE but not in Firefox... any ideas?

You can't - the fact that that happens in IE is generally considered a bug.

And it's an accessibility no-no at any rate.