The strange truth about firbolg names
Here is the wrinkle that makes firbolgs unique: by canon lore, firbolgs do not use names among their own kind. Inside the clan, a firbolg is known by deeds and presence, and a name would be redundant among family who know you completely. Names only become necessary when a firbolg leaves the forest, so every named firbolg is, in a sense, wearing travel clothes. Most adopt Elvish names, since elves are the neighbors firbolgs trust most.
Where the names in this generator come from
The results above lean on Elvish forms, gentle and woodland-flavored, plus nature epithets that work the way firbolg reputations do inside a clan. A firbolg introduced as Caelum Mossmantle is really carrying a translation: this is roughly what my people would say about me. Soft consonants, long vowels, and earth-and-growing-thing surnames fit the firbolg better than anything martial. If a result sounds too sharp, drop a syllable and it will settle.
Roleplaying the borrowed name
A firbolg's name is a mask they chose, which is a gift for roleplay. Maybe your druid picked her name five minutes before entering her first human town and regrets the choice. Maybe your cleric answers to three different names in three different villages and sees no contradiction. The name your firbolg gives the party may also differ from how they think of themselves, which makes even a simple introduction scene worth playing out. Roll until something feels borrowed in the right way.
What firbolgs call each other
Inside the clan, firbolgs identify each other by trait and deed: the one who mends, the one who walks at night, the one the foxes follow. If you want your firbolg's outward name to feel rooted, translate one of those inner descriptions into the surname. A firbolg called Galanthel Foxfriend has simply put her clan's private description into Common, and that is exactly how the culture works.
Sample names from this generator's firbolg list: Aenon, Ailill, Aoife, Brigid, with clan or family names like Mossheart and Stagward. Roll above for the full range, and click any result to copy it.