Hi tmakaro, thanks for following up.
Filling credit cards and identity information with 1Password in Firefox is not currently supported and the changes required to do so are out of our hands. With other supported browsers such as Brave, Chrome, DuckDuckGo, Edge, Samsung Internet and Vivaldi, you can fill identities and cards in more and more places with 1Password.
I was actually looking at Firefox add-ons in the nightly earlier today. I wouldn't expect that the 1Password extension would work as is in Firefox mobile, but I think it would be a great space to expand into in the future. Thanks for sharing this with us!