The look of the U4E BFG changes in almost every version, but none of them looks anything like the original. It's a stuffed doll with a nuclear bomb inside. You can throw it with primary fire, and it'll run around until it explodes on its own, or hits someone. It also detonates any of your dolls currently running around. The classic flamethrower.
Primary fire shoots the selected mode, either classic flames, or a stream of ice.
Alternate fire switches between modes. They have a few seconds to pick up health and stop the effects, but after that, they explode in a Redeemer-like blast, killing them and almost anyone around them. The FBR has a low rate of fire, but even that doesn't make up for its power. Alternate fire shoots one rocket. If you move forward while shooting, the rockets will probably hit you, so try moving backwards.