drivebelt slip off

I had the unfortunate experience of my drive belt coming off completly while drive on the thruway...I lost all steering, a/c luckily for me there was a gas station nearby, and I got it off the thruway, I'm thinkin somethin is wrong or damaged with the belt tensioner, I got the belt back on and drove it home but part of the belt is not centered on one of the pulleys ( on the 99 firebird V6 it's above and on the right side of the alternator... got any ideas as to why this happened???

Start by making sure you have the right length belt and number of ribs. if thats correct then check the pulleys and make sure that they are all in line. Then check the pulleys to see if they seem loose. A bad bearing will cause a pulley ro ride incorrectly and could cause the belt to pull in a direction.