[RPG] How do dwarves get their food

Plants and Fungi

Not surprisingly there are edible mushrooms and plants that adapted to the Underdark. There are hundreds of different wines produced from fungi. "Spiderblood" is a one of the most expensive and seasoned with spider's venom.

• Barrelstalk is a fungus as big as a tree. Only the inner flesh is edible. In the centre there is a large reservoir containing water.
• Bluecap is an inedible fungus, whose spores can be ground into flour and made into bread, commonly called "sporebread".
• Firelichen of pale orangish color is used to produce a spicy paste put onto sporebread, liquor or to season soup or stew.
• Ripplebark is a fungus resembling a pile of rotting flesh. It is edible raw, however after preparation it has better taste.
• Trillimac is a fungus with broad gray-green cap and a light gray stalk. The stalk can be cleaned, soaked and dried to produce bread-like food.
• Water orb is a bulbous fungus that grows in shallow water. It's structure is similar to sponge, soaked with water. It is edible, but rater chewy and tasteless.
• Zurkhwood is enormous mushroom and a source of food and timber. Because of hard and woody consistency it must be properly cooked to be edible. Its spores are nurtritious and may be eaten raw.

Animals

• Bats are very common in the whole of the Underdark, and includes regular, dire and even weirder species.
• Fish are mostly very small, pale and harmless. In greater bodies of water more dangerous versions of subterranean fish may be found, e.g. sharks.
• Rothé are common livestock very similar to muskoxes.
• Vermin are the most common source of food in the Underdark. Some of them are bred like livestock, mostly beetles and crickets.

The source materials are:

• Underdark, the D&D 3.5e campaign accessory

  

• Out of the Abyss, a D&D 5e adventure

  

The Princes of the Apocalypse adventure takes place in the Forgotten Realms by default, so I'm going to base my answer on that setting. If your DM has significantly changed that default, some of the following may not apply.

The 5th edition Player's Handbook has a sidebar for each of the common races with a brief description of the common attitude of that race towards each of the other basic races. This helps in the case of elves, halflings, and humans, but not firbolgs.

The Sword Coast Adventurer's Guide has a section on each of the races from the PH, and may give some insight into the general attitude of dwarves, but the book focuses less on interracial interaction and probably won't help much.

Volo's Guide to Monsters is the only 5e source of information about firbolgs that we have. Unfortunately, the firbolg section only mentions elves and gnomes, if I remember correctly, and makes no reference to dwarves.

Mordenkainen's Tome of Foes has an entire chapter about dwarves, including significant treatment of dwarven attitudes and outlooks. While it doesn't detail their views on every race, it could provide enough foundational material to let you make significantly more informed guesses.

As for pre-5e materials, I'm afraid I'm only personally familiar with one novel that has a firbolg, one of the Cleric Quintet novels by R. A. Salvatore. In it, the protagonist and his two dwarven friends run into a firbolg. The dwarves have an inkling of dislike for the firbolg due to the race's relation to giants, but they remind themselves that firbolgs had also sided with dwarves in the past against evil giants, and so were willing to look past the lineage and treat him about the same as they would a human.

It is important to note that while dwarves have been portrayed fairly consistently in Forgotten Realms materials for some time, firbolgs underwent significant changes in 5e compared to older editions, and looking at old material for specific interracial preconceptions may be less useful than extrapolating details from current general information on 5e lore, such as that found in Mordenkainen's Tome of Foes.