Does a 5.7L have the same fuel tank as 3.8L in 1996 Pontiac firebird?

No, the 1996 Pontiac Firebird did not have the same fuel tank size across all engine options. While the exact capacity might vary slightly depending on trim level and options, the 5.7L (typically the LT1 or LS1 depending on the exact year within '96) generally had a larger fuel tank than the 3.8L V6. You'll need to consult a 1996 Pontiac Firebird owner's manual or a reliable parts website with specifications for the particular year and trim to be certain of the exact fuel tank capacity for each engine.