Passenger compartment air filter, I would assume.

http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=general&Number=193161&Forum=general&Words=passenger%20filter&Match=And&Searchpage=0&Limit=25&Old=allposts&Main=192985&Search=true#Post193161

OR- mice in there, chewing something up. It doesn't seem like it would be a filter just falling apart. You could also blow out the slot- maybe it's just full of old dust and crud. I'l take a look in the air filter compartment in any case, I bet you find the answer there.

-Philip Maynard '95 Contour [71 STS | Track Whore] '97 Miata [71 ES | Boulevard Pimp] 2006 autocross results