That's one of the reasons that I don't like FF, it's very strict about what it displays and if you have any errors (though you have quite the bundle of them

) it will break things that IE will display. Code purists love FF, general users like it for safety but don't care about code purity, and we web designers - like it or hate it, it's a necessary evil and you have to check your site in all three "big" browsers (IE/FF/Opera) any time you do anything.
It is indeed a pain in the ass.