There is such a difference between how Firefox renders code versus how IE 4 renders that I think it would be best if you used a separate stylesheet for IE4, IE 5, and maybe IE6. I doubt very much you will get that footer fixed with one set of code.

By having separate stylesheets you can set whatever height you need for IE 4 to make it appear 30px high.

Have you checked your stats lately to see how many people are still using IE4? I'd be curious. My stats this morning show 3% of people.