The strength HM can be obtained by giving the warden his gold teeth (which can be found on the way to the secret house of the safari zone)
If you give the warden his teeth back you get HM04 which contains Strength.
Pushing boulders is a feature that can only be done with HM04 Strength. HM04 is received after you find the Safari Warden's Gold Teeth in the Safari Zone. When you have the Gold Teeth, give it to the Safari Warden and he will reward you with HM04.
There Are Only 7 HM's In Pokemon Firered. Hm01- Cut Hm02- Fly Hm03- Surf Hm04- Strength Hm05- Flash Hm06- Rock Smash Hm07- Waterfall
you need to get the gold teeth, which is in the Safari Zone in area 3. After you get the teeth, take it to the warden, which is to the right of the Pokemon center. Give it to him and he will give you HM04, "strength."
Hm01 Cut, Hm02 Fly, Hm03 Surf, Hm04 Strength, Hm05 Flash, Hm06 Rock smash, Hm07 Waterfall.
HM04 Strength I believe that the warden of the Safari Zone in Fuchsia City gives it to you after you return his teeth but that might be surf... Haven't played it in forever xD
lost tower
There are eight of them. HM01 Cut HM02 Fly HM03 Surf HM04 Strength HM05 Flash HM06 Rock Smash HM07 Waterfall
In Pokemon firered you can't, but in Pokemon leafgreen you can get it then trade it to Pokemon firered.
There is no Pokemon convention in firered.
HM04
In the top floor of the lost tower