guys, I thank you so much for the help!!
It really was because of the doctype ... never considered the doctype as much important, now i do.
I used XHTML 1.0 TRADITIONAL, cause STRICT caused problems with the tables in FF, however
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
@smashingred: No, you’ve not been little rude ... you’ve been exactly right, this page is a tag soup because of several reasons ... I just implementet MODx afterwards and was somehow too lazy to re-code everything ... just wanted the side to quickly get working; A mistake maybe, but my lecture for the next few weeks will be XML and w3 standards
so, thanks again!