*shrug* A mathematical ball can be 2D, but I'm pretty sure it's not he intended meaning. A 2-ball is the interior of a circle, just as a 3-ball is the interior of a sphere. (Mathematically speaking, a ball is a strict inequality -- they don't include the boundary.)
Fireball expands in all dimensions. If you really care that much, there was a designer talking about it on the Eric's news page (the Black Pages now?) shortly before 3E's release.