Try asking on mozilla forums they may know more information about your problem. Or hopfully some of the peeps here maybey able to help.
@urie and HMonk Thanks both of you for your response. The problem has been resolved, in a big percentage, by setting up the whole site from the very begining and taking into account some prerequisites and making a few compromises. Finally I believe that that the main effort would be, the site, to be appear corectly in IE and secondarelly in FireFox, because IE is used by the big majority. That is my oppinion and I hope not to be wrong. Thanks once more.