the original "firebird" concept vehicle existed in the 50's. it was only an engineering/design excercise. it was not intended to be a production car in any way. it was an airplane inspired vehicle that didn't even have a V8 in it.
the only portion of the orignal that lived on is the small model on top of the trophy given out to the daytona 500 winner each year.
pontiac did have another concept vehicle that used the firebird name in the early 60's. the under pinnings were all vette on a revised chassis and with a 389inch experimental pontiac motor. it was a largely unrealistic car because of all that delorean designed into it. it would have outperformed the vette, but at nearly twice the price tag. there simply wasn't a market for it.
delorean and pontiac got on board with the f-body concept after the camaro had begun design. GM's use of unibody construction allowed for being somewhat modular in construction. this gave pontiac the opprtunity to drop their power plants into the platform and use their own styling department which was much mroe aggressive than the chevrolet guys at the time.
pontiac was much more in the "muscle car" spirit at teh begining. they had test mules with every engine and trans combo in their line up tested and ready to go before the first car ever even hit a dealership. they had the "big engine, little car" thing down to a science.
