Interesting. A bit of a departure from depictions in earlier editions. Much more nature oriented...seems they're playing up the fey aspect based on the Celtic origins of the race.
Interesting wording on Hidden Step, though. It makes you invisible "...until you attack, make a damage roll, or force someone to make a saving throw." This is different from all other invisibility effects. I wonder why.
Why put in a race that diverges so significantly from the original other than to pander to nostalgia? Why not create new races or utilise the plethora of other available options if they want to fill a book?
Firbolg have been 10 ft. tall since 1e and in every other edition. They've always been of a large size and, in fact, one of their special abilities has always been to shrink down a size. It's really just absurd to shoehorn them into a medium build so that they'll be a playable race.
So why have it as a playable race at all then? There are hundreds of other races they could've chosen to make playable. There was no burning need to have firbolgs be a playable race. But to make them one, they had to so radically alter them from their well known and established lineage that they no longer resemble the original which makes the entire exercise pointless other than to play upon nostalgia. They could've simply called it some other thing and made it an entirely new race if they were that keen on introducing another goliath-like race.It allows for firbolgs to actually fit in with the standard adventuring party.
And people laughed when I suggested Firbolgs would fit in better as a player race as compared to Goliaths.