This paper proposes a technique of firmware development based on Multi-valued Decision Diagrams (MDDs). Evaluation of multiple-output Boolean functions is faster than the one using Binary Decision Diagrams (BDDs) and has a small memory footprint often required in embedded systems. A micro-programmed controller that firmware runs on is supposed to support multi-way branching in hardware, whose implementation is known. A novel heuristic technique of a sub-optimal multivalued MDD synthesis is presented and a specific condition for spatial efficiency of MDD-based firmware is derived. The method is illustrated on practical examples. It may be quite useful for development of embedded microcontroller firmware as well as for fast digital system simulation.