When people think of a Gibson Firebird they think of the reverse model. The original Non-reverse birds featured P-90's, in line tuners and the slider switch. You tend to see most of them in either Sunburst or refinished natural looking walnut. The original reverse Firebirds featured banjo tuners, mini HB's and the standard Gibson toggle found on their other guitars. The reverses also tended to come more often in custom colors which makes them more desirable to collectors due to their "custom" nature and the limited supply of these guitars out there. The custom colors like Pelham Blue, Inverness Green, etc. tended to be put on the upper model Firebirds (V's and VII's) but also was available to Firebird I and III's. The Reverses also did come in natural sunburst as well. Most notable Firebird players became famous using Reverse models so I think this also has an impact on the desirability of these instruments.
The two models have noticeable differences and thus have noticeable sound differences. So it comes down to player preference which model is your favorite. Personally I own a Gibson USA Firebird V Reverse with the mini HB's and really enjoy this guitar. The mini HB's truly make it a unique guitar because nothing else currently on the Gibson line features them. Not to say I don't like P-90's, I own a Gibson SG with P-90's another great guitar. But when it comes to the birds I am a reverse man through and through.