Look at the box for the letter of firmware: 
(image from MODshark.com) No Letter = 1.50 (North-American release) Confirmed
A = 1.50
B = 1.51
C = 1.52
D = 1.52 Unconfirmed (multiple version reports)
E = 1.52
F = 2.00
G = 2.01
H = 2.50
I = 2.60
J = 2.60 Unconfirmed (multiple version reports)
K = 2.71 Unconfirmed (multiple version reports)
L = 2.80
M = 3.00 Unconfirmed (multiple version reports)
N = 3.01
O = 3.02
P = 3.03
All of the above firmwares can be downgraded to 1.50 then to custom firmware at wish. Use the tutorial section to see how to downgrade the PSP system softwares.