Pontiac 2001 Firebird Owner's Manual page 258

The cap is located behind a hinged door on the driver's
side of your vehicle.
6-6
While refueling, hang the cap from the hook on the
hinge. The lip of the cap will fit into the groove on
the hook.
To remove the cap, turn it slowly to the left
(counterclockwise). The cap has a spring in it; if you let
go of the cap too soon, it will spring back to the right.