Like Google, Facebook, Apple... Make it possible for websites to make a Login with Firefox option available so we have a trusted privacy preserving option given to us (and give Firefox some visibility)
If I can also pay with Firefox account not only to sign in, then I will be a Firefox member proud to be. Yet, we may need also to create an email account like AT firefox.com to book my ticket for the Moon as wealthy as the first man who will land on the Mars Planet in 2030, we are only 6 years left before the lift off.
On the sole condition that no exchange of data is possible between the services. If I log in with a Firefox account, there is no question that the organization will collect my personal information, but only my public account information.
And in the end, I'm not sure it's possible.
The advantage of having multiple accounts is that it is more difficult to track you.
@Erwan62I am not entirely familiar with how 'Login with ____' services work, but I think Firefox could make an agreement with companies or have their 'Login with Firefox' option not allow for exchange of data. It would just be a login service, after all.