Fire Space was a pocket dimension that Master Megatron created using the power of Unicron. It is essentially a portion of his internal universe, but bathed in fire and resembling the inside of a volcano. The term is heavily used in the japanese show Galaxy Force. It also exists in Cybertron in identical fashion.
Galaxy Force
Fire Space is depicted as a fire dimension, resembling the inside of a volcano, and under Master Megatron's personal control. The walls of Fire Space can be used as visionary portals, allowing all inside to observe and spy on whatever they choose.
Escaping to Fire Space after obtaining the Chip Square, Sonic Convoy pursued Master Megatron and fought him to a standstill. Sonic Convoy was able to win back the Chip Square and escape the fire dimension. As Master Megatron began to emerge in pursuit, the Omega Lock and Vector Prime's sword were used to colapse Fire Space around him, resulting in Master Megatron's first demise.
Although Fire Space was colapsed, it would be created again by Master Megatron on Gigalonia after his defeat at the hands of Megalo Convoy.
A portion of Fire Space is currently being controlled and accessed by Super Starscream, after losing the battle with Master Galvatron.
Cybertron
The existence of Fire Space is essentially identical in the Cybertron material.