Most FB's are easy to trap. Use bridges (raised bridges are indestructible, although heavy FB's can prevent their closing) with a bait item or animal or hippie-on-a-rope between them. If you are feeling lucky, you can even have this corridor lead into your fort (you won't need the bait animal in this case). Regardless, when the FB paths into your corridor, close both of the bridges. You'll have succeeded in "trapping" it. Then you can use other bridge-corridors and furniture/puppy snacks to move it elsewhere in your fort. I like to keep one as a pet "rancor" that I can feed prisoners. Web-slinging ones can be used in silk farms (the silk has no value multiplier though). The syndrome-spitting ones can be used to coat your weapons in poison! Just make sure everyone is wearing gloves... but seriously, it works. Set it up like a silk farm, but put a (bin-less) weapon stockpile behind the sacrifice. The syndrome-spit will splatter over your weapons. Voila! Your militia will wield blades that paralyze the hearts of any creature they touch!
For your flame-beast, you can make a pill-box turret out of one. Get him corralled in a fortification-lined room near the entrance of your fort. Keep the fortifiactions blocked off with raised bridges (you basically never build bridges to just be bridges in this game...). When a siege arrives, lower the bridges and let the FB toss fireballs at the goblins. If nothing else he'll set the grass on fire... which should set the beak dogs, trolls, goblins, etc on fire too. Fun times! As mentioned, they are fragile though. Close the bridges if a skilled bowmen gets too close. At point-blank range, fortifications can be shot through. With high enough skill, they can be shot through with no penalty at point-blank.