Were is the desert in Pokemon FireRed?

There is no official "desert" in Pokemon FireRed. However, you can consider the Route 30 area, specifically the Tanoby Sands, as a desert-like environment. It has a sandy landscape and is home to Pokemon like Sandslash and Cacturne, which are associated with deserts.

While not a dedicated desert location, Route 30 provides a similar feel and features desert-themed Pokemon.