Doctyping is it needed?

Hey I have been pulling my hair out over a javascript scroller that would not work in FF.
I have just removed

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">

and now it goes.

So really what I want to know is will there be big problems if i just leave the doctype out of my html document??

iv also tried all the other strict, loose etc, etc and they all do the same thing.

wierd…