Are you using the add-on?

In the add-on settings, you can toggle OFF the "Show Relay icon in email fields on websites" to disable the in email input fields.

Or in Firefox settings, under Privacy & Security -> Passwords, you can uncheck "Suggest Firefox Relay email masks to protect your email address"

In the extension button pop-up, you should see a gear icon for the Settings. In there, you can toggle off the "Show Relay icon in email fields on websites" If you don't see the extension button in the top of Firefox, you should be able to see it if you click the puzzle-piece Extensions icon.