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. It is also used by the Sphinx.
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.
With the Sphinx, the fireballs can be destroyed by Iceberg Lettuce. It also burns plants instead of squashing them.