I couldn't find the article I read recently about it, but here's some reference to it.
Tony Bacon suggests in
Flying V, Explorer, Firebird: An Odd–Shaped History of Gibson’s Weird Electric Guitars that Fender may have threatened a suit,
but Gibson likely just used those rumors an excuse to pull the original Firebird and update its poor design. He states that Fender was busy around that time to get patents on the Jazzmaster/Jaguar and that they didn't file a lawsuit at all. But they did print ads about the Jag/Jazzmaster with 'the most copied design around' or something with a similar meaning. He also states Gibson made the nonrev because it just was cheaper to make. A bit less radical and no neck through.
from the Gibson Firebird Wiki page:
Gibson's line of
Thunderbird basses is rooted in the design of the Firebird, and uses even Roman numerals ("II" and "IV") to distinguish it. 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.
It always seemed like a stretch to me - as in, do you believe a rumor or your own two eyes! If anything - the NR looked more like a Fender bass...