You can buy an acid to clean off bricks, but be sure to read the instructions before you start!
Try using TSP or Krud cutter with micro fiber cloths and one of those big sponges in the contractor area.
For the outside I used Krud Kutter, if the inside, I used professionals.
There are lots of posts =clean%7Cautocomplete%7C0&term_meta=brick%7Cautocomplete%7C0&term_meta=fireplace%7Cautocomplete%7C0]here that can help you
Here's a post that might help you - https://www.wikihow.life/Clean-Fireplace-Bricks
Terri....I found a spray at Lowe's for my brick fireplace, used lots of newspapers..pretty messy but worked well for me.