Firefox browser not recognizing css; Explorer is – what gives?

can anyone give me a hint as to why Firefox doesn’t display an html-formatted page…but Explorer is?

everything was fine up until three days ago – i’ve ONLY changed one setting in my control panel since then (file association for mht & mhtml) …and have changed back to see if this was the problem but it doesn’t (and shouldn’t) make a difference.

any ideas..?

thx in advance!