You may want to try Zen Browser - a Firefox fork that includes a chrome-like multi-profile UI.

Proton passkeys are stored in a password manager, which he specifically calls out.

If you have a password managed and know how to use it, you’re already a lot less susceptible to the problem that passkeys are trying to solve.

Personally, I think passkeys are great for tech-savvy users, but I wouldn’t dream of recommending them to non tech-savvy people. Password managers are still used by the minority, that needs to be fixed before passkeys are useful.