Wash and scrub the mussels in cold water.
Bring 2 cups of water to a boil in a large heavy pot. Add the mussels and cover the pot.* Steam for 6 to 8 minutes or until the mussels open. Discard any mussels that didn't open. Remove from the heat and set aside.
Cut the fish into 1-inch thick slices.
Heat the oil in a large pot. Add the garlic, onions, tomatoes, fennel, saffron, salt, and pepper. Add the mussel broth to the pot and 2 tablespoons of butter. Add the boiling water and clam juice. Bring to a rolling boil, add the crabs and fish, and continue to boil for 6 to 8 minutes. Add the mussels along with the shrimp, and boil for an additional 5 minutes.
Slice the French bread, butter it, and toast lightly. Serve the bouillabaisse in large individual bowls lined with toasted French bread; pile in the seafood and ladle the broth over it. Top each bowl with 1 tablespoon of butter. Pass the remaining bread at the table.
If you like a spicier soup you may add a couple of whole hot peppers to the pot at the start of cooking.