If pages won't show in Firefox, they are wrong. Think about browsers as spell-checkers. Firefox goes through and checks the spelling of every word, and then either passes the document or fails the document. IE goes through and checks every word, and then if some are misspelled, it says "eh, no big deal" and passes the document. That's a pretty lame metaphor, but any code that IE will interpret and Firefox will not is madeup bullshit code, and the developers should be punished.