You can setup/or change your passcode by going to Settings and Face ID & Passcode. If you can't do this, you need to asked the question on the Apple support site. If the phone is previously owned you'll need to contact the previous owner for the code.

The passcode requested by the firefox app is different from my iPhone's passcode. I still have access to my old phone and it always had a 4 digit numerical passcode, but for some reason, the firefox app asks for 6 digits. I removed/altered the passcode on my new and old phone and I still cannot access my logins and passwords without some type of bio-authentication. I attached screenshots of my lock screen and firefox app prompt from my old and new phone.

Got it. Does this help?

https://support.mozilla.org/en-US/kb/set-passcode-and-touch-id-firefox

Unfortunately, no. To turn the firefox passcode off or change it, I need the 6 digit passcode. Currently, I can't find any avenues to reset that passcode if I forgot it.

Didn’t think it would work but something to try.