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.
I had to go back and check the changling and I have to say, the Firbolg is not better than them at disguise. For the changeling, it is a physical change; the firbolg's is an illusion and can be seen through with an intelligence/investigation check. It also only lasts an hour and requires the firbolg to have at least a short rest to recharge it. The changling is a physical change, so it can't be seen through. It is an at will action to change, they do not have to rest to recharge, and it lasts essentially forever. They also have proficiency in deception as a racial ability.
A changeling is far superior at disguise than a firbolg for anything long term or with preparation and I would prefer a changeling as an infiltrator.
The firbolg's disguise self is useful only in that it can also change the appearance of their clothes with their new disguise. However, they are unable to change their appearance again during the duration of the spell.
Off the top of my head I can think of two things.
Large weapons (significant damage increase*) and ability to grapple huge creatures (significantly reducing the number of grapple-immune foes).
*) I can't actually find a rule that governs how damage changes with the size of weapons. There's nothing in the Equipment chapter of the PHB. The MM talks about how size changes your hit dice, but not your weapon dice. The DMG does say you double the damage dice for a weapon wielded by a Large creature, but it is far from obvious this should apply to player characters. After all, the Enlarge spell makes PCs Large but only allows +1d4 damage.
And on the downside (just as you say), the inability to follow the rest of the party through cramped environments. (Of course this is not a big deal if you have an ability to shrink, or if your DM allows access to those Underdark fungus detailed in Out of the Abyss). I guess another downside (at least for the first few levels) is that you can't actually block goblins and other small creatures.