Trim off feathery fennel tops; rinse, drain, wrap airtight, and chill. Trim off and discard base of fennel head, any bruises, and tough stalks. Rinse and thinly slice fennel head.
Trim tough tops and roots from leeks; pull off coarse outer layers. Cut tender sections of leeks in half lengthwise, rinse well, and thinly slice.
In an 8- to 10-quart pan, combine fennel, leeks, garlic, and oil. Stir often over medium-high heat until vegetables are lightly browned, 8 to 10 minutes.
Add tomatoes with juice, broth, clam juice, wine, bay leaves, thyme, orange peel, and saffron. Cover and bring to a boil over high heat.
Meanwhile, with scissors, cut fins and sharp spines from lobster tails. Also cut through top shells lengthwise down the center. Set each tail, underside down, on a board. With a heavy knife, slice tails in half lengthwise through cuts in shells. Rinse lobster tails, halibut, shrimp, and scallops.
Add lobster, halibut, and shrimp to tomato mixture. Discard any clams or mussels that do not close when shells are tapped. Add clams and mussels to pan; cover and cook 5 minutes. Add scallops. Cover and simmer until mussels and clams pop open, 3 to 5 more minutes.
Transfer cioppino to a tureen or ladle into wide bowls. Garnish with green fennel tops.