The basic land types are Plains, Island, Swamp, Mountain, and Forest. Land types other than basic land types (such as Desert) don't contribute to domain abilities.

Increasing the target creature's power in response to Radha's Firebrand's first ability can cause it to be an illegal target for the ability. However, once the ability has resolved, it won't be able to block even if its power becomes greater than Radha's Firebrand's power.

Domain abilities count the number of basic land types among lands you control, not how many lands you control or how many of any type.