1998 Pontiac Firebird serpinteen belt jumps off tentioner

First of all.......you need to check for bent or damaged pulleys. Look closely for any pulleys that may be angled. The best way to do that is take the belt off, spin the pulleys and watch for a considerable ammount of wobble. Next.......check the tensioner. If its not putting enough tension on the belt, it could cause the belt to jump off. When the belt is on, how far is the tensioner moving? If the tensioner isnt applying enough pressure against the belt, it could be letting the belt pull the tensioner enough to let the belt jump off. Most likely from what Ive seen in the past, the belt itself might be stretched. Is the belt old? Does it have little cracks on the inside? Is it worn on the edges? Have you had any components replaced that use the belt(water pump, power steering ect...)? These are all questions you need to ask yourself while inspecting. Your serp belt is a very important part. If it goes.....so does just about everything needed to run your car. Any new parts that were installed incorrectly like the above mentioned, could be the source. If thats not applicable, focus on tension and the belt itself. Those would be the 2 main suspects. Start off cheap and try a new belt if the tensioner is strong and everything else looks good. If the old one didnt break......keep it as an emergency spare. Those are always great to have in case of emergency. Good luck and let me know how things go. :)