Apologies if this has been reported already. Searching for 'Firefox' gives a lot of false positives ;-)
The Firefox account page (for syncing Firefox between devices etc.) doesn't auto-fill. They also have 2FA and the OTP also doesn't auto-fill on the subsequent page.
Main sign-in page: https://accounts.firefox.com/signin
1Password Version: 7.7.1.BETA-4 (70701004)
Extension Version: 1.23.2
OS Version: 10.15.7
Sync Type: Not Provided