Wiki basically has the whole scoop:
The Gibson Guitar Corporation released several new styles during the 1950s to compete with Fender's solid-body instruments, such as the Telecasterand Stratocaster. After success with the Les Paul in the 1950s, Gibson's popularity began to wane in the 1960s...
The president of Gibson, Ted McCarty, hired car designer Ray Dietrich to design a guitar that would have popular appeal. Under Dietrich, the Firebird took on the lines of mid-50s car tailfins. Dietrich took the Explorer design and rounded the edges. The most unusual aspect is that the guitar is "backward" in that the right-hand (treble) horn of the body is longer than the other. Thus, the original Firebirds were unofficially referred to as "reverse". The Firebird looks like an Explorer with softer "points," or maybe a reverse Fender Jaguar or Jazzmaster.
Explorer and Firebird side by side:
The Firebird line went on sale in mid-1963 with four models distinguished by pickup and tailpiece configurations (see below). Unlike the Les Paul and SG line, which used the terms "Junior", "Special", "Standard" and "Custom", the Firebird used the Roman numerals "I", "III", "V" and "VII"....
From 1965 to 1969, Gibson introduced "non-reverse" models after failing to achieve marketing success with the unusual reverse-body design. Gibson had also received complaints from Fender that the Firebird headstock mirrored the Stratocaster and that the body violated Fender's design patents, with Fender threatening a lawsuit. The "non-reverse" body is a more standard double-cutaway design, with the bass horn being longer than the treble horn and the headstock having the tuners mounted on the bass side....
After a few years of disappointing sales, the "non-reverse" line was dropped. "Reverse" body Firebirds were first reissued in 1972, with a commemorative "Bicentennial" model released in 1976 which was made available in a variety of finishes including black, vintage white, natural, and the traditional sunburst. The bicentennial model was also distinguished by gold hardware and a red-white-blue logo on the white pickguard.
1965 Gibson Firebird I Non-Reverse: