Traditionally Firbolgs in FR are big human looking giant kin, with red hair.
But what they did in VGTMs is take the wood giant and rename it Firbolg, messing up the lore of both in 5e's often needlessly sloppy way. Seriously look up Wood Giants from previous editions and the appearance and abilities are very simular to the 5e Firbolgs. I love the race, but hate that they needlessly misnamed them.
Voadkyn
5e Firgbolg are Voadkyn aka Wood Giants, right down to the disguise self spell, which Firbolgs never traditionally got.
Why WotC doesn't just call them Voadkyn and instead messed up the lore by taking the name of a different kind of Giant and applying it to Voadkyn. In fact from now on I refuse to call them Firbolgs, they aren't, I'm calling them Voadkyn.