Fireball is a giant sphere of what seems to be molten metal or magma. It is one of the Zombot's attacks used in Level 5-10 and the mini-game Dr. Zomboss's Revenge, and is released from the Zombot's mouth.
During the mini-game, the Zombot may move its head down to pick a lane on which to place the projectile. If the Zombot's eyes turn red, it will shoot a fireball. After a few seconds, the fireball will roll down the lane and squish every plant it touches until it is destroyed or it rolls off the end of the lane; it will also crush any Roof Cleaners in the row. Its weakness is the ice created by Ice-shrooms. It is easier to stop than the Iceball, because Ice-shrooms have an unlimited range, while the only counter to the Iceball is the Jalapeno, which can cover only one lane.
With the Sphinx, the fireballs can be destroyed by Iceberg Lettuce. It also burns plants instead of squashing them.