HM06 Rock Smash Guide - Pokemon Fire Red & Leaf Green
HM06 Rock Smash is obtained from an old man inside Ember Spa which is found on Kindle Road. You'll find Kindle Road on Island 1, it leads north out of the city to Mt. Ember which is where you'll find Moltres. The cave you're looking for which has the old man inside of it is just before the water that you surf across to reach Mt. Ember.
If you're unsure of where I am talking about, use my screen shot above for some guidance. Once you're inside of the Ember Spa head to the northern portion of the area where you'll find the old man, talk to him and you'll receive HM06 Rock Smash.