Firefox's First Party Isolation can break some websites that might rely on third-party authentication. This may be acceptable for Tor users, but not regular users, which is why First Party Isolation is still off by default in Firefox. Mozilla is actively testing a variation called Dynamic First Party Isolation that will support those workflows.

For more details, here is the Firefox bug report:

https://bugzilla.mozilla.org/show_bug.cgi?id=1549587

To enable Dynamic First Party Isolation: in Firefox Nightly's about:preferences#privacy, select "Custom" Enhanced Tracking Protection and "Cookies" = "Cross-site and social media trackers, and isolate remaining cookies". Or just set `network.cookie.cookieBehavior` pref = 5 in about:config.