I have a 91 firebird 350. The guy I bought the car from says that he put a suburban transmission in it. My question is why? and how does this effect my speed/acceleration. Thanks

"Suburban transmission" could mean anything. They came with TH350's, TH400's, TH700R4's, 4L60E's and I think 4L80E's, too over the years. The 700R4 in a Suburban and a 700R4 in an f-body have the exact same gears and internal parts, so there's no benefit or strength issue. He probably put it in because that's what he had available to him at the time.

thank you

Governors are different. F-Body uses very light weights and provides later shifts. Trucks use heavy weights and tend to shift much earlier.