The plan was only to kill off third-party cookies, not first-party so being able to log into stuff (and stay logged in) was not going to be affected. Most other browsers have already blocked or limited third-party cookies but most other browsers aren’t owned by a company that runs a dominant ad-tech business, so they can just make those changes without consulting anyone.

Also, it looks like there might be some kind of standard for federated login being worked on but I haven’t really investigated it: developer.mozilla.org/en-US/docs/Web/…/FedCM_API