Can you go to other regions in Pokemon firered?

If your referring to Jhoto, Hoenn, or Sinnoh then no.

But you can go to 7 Islands(9 if you have certain items from Nintendo Events).

You can get to the first 3 after you beat the 7th Gym Leader, Blain. You can also catch Moltres on the first island. And the other 4 once you complete the Kanto Pokedex and receive the NationalDex. On the other 4 Islands you can catch Pokemon from the Jhoto and Hoenn regions.