Ever since I read their stats in the Monster Manual 2, I have loved these giant, good-aligned, Celtic-styled giants. In a game full of evil, malign, savage and otherwise out to get you monsters, especially among the giants, these beardy goliaths are a nice break from the norm.
|Pic from CoolMiniOrNot.com|
Although they can still pose a threat to adventuring parties, they often have more detailed motives for being so. Instead of hurling boulders at you simply for the sheer sport of it or because they want to grind their bones to make their bread. A Firbolg is more likely to attack a party because they are invading his lands, have taken some of his stock or wronged his reputation or kinfolk somehow.
As much as they can be foe, Firbolg also offer a good chance for something special in a fantasy game - a giant on your side. Being good-aligned, they can just as easily be allies to a party as they can be enemies. In my own games, I have had a Firbolg seek aid from the party, serve as a guide and in one game there was even a Firbolg PC (I was pandering to a player, I'll admit it).