Mozilla did a study with user testing. Breaks lots of federated logins / Single-sign on solutions. Too much breakage to enable it by default. If you confuse use

Mozilla did a study with user testing. Breaks lots of federated logins / Single-sign on solutions. Too much breakage to enable it by default. If you confuse users with this, they switch to GChrome and all you did for them is a privacy disservice. There's a First Party Isolation add-on though if you want a reachable toggle button.