What's the best way to remove musty odor from stored leather furniture

Let it sit outside (in the shade) to air out. Or use white vinegar and dilute 50/50 with water to clean the leather. It will clean and rid it of any smells. Put solution in a spray bottle, spray over all sides and let sit for 10 minutes then wipe away. If it still smells, repeat the process. Follow with leather conditioner if desire.

I would try stuffing dryer sheets with a couple of drops of your favorite essential oil in the cushions and see if that will help.

I'd air it outside, but clean it with a rag, warm water mixed with Pinesol 'blue'. It has a wonderful fresh scent. And I agree with Kiggy, put a few dryer sheets between the cushions. Maybe a 'rinse' with warm water and fabric softener. Best of luck.

I say sunlight is the answer.....try baking soda in a container with holes in it to absorb some of the odor....there is leather cleaners and conditioners in bottles that might help.....make sure the underneath of the couch is washed down good also and all other parts..