Follow the steps below to disable Firefox password manager:
- Tap on the three dots menu and open Settings.
- Select Logins and passwords.
- Tap Save logins and passwords.
- Choose Never save.
- Go back by tapping the arrow at the top and switch the toggle Autofill in Firefox to off.
- Go back and select Autofill.
- Switch the toggles Save and autofill addresses and Save and autofill cards to off.