How do you get Onix in Pokemon FireRed?

You can't find Onix in Pokemon FireRed! It's not available in that game.

However, you can trade an Onix from another Pokemon game into FireRed.

Here's how to get an Onix in other Pokemon games:

* Generation 1 (Red, Blue, Yellow): You can find Onix in the Rock Tunnel.

* Generation 2 (Gold, Silver, Crystal): You can find Onix in various locations, including the Ilex Forest and the Rock Tunnel.

* Generation 3 (Ruby, Sapphire, Emerald): Onix is found in various locations, including the Granite Cave and the Victory Road.

If you have a way to trade Pokemon from other generations to FireRed, then you can get an Onix that way!