For years in the past, developing web apps was a constant case of if (IE) do_something_else_than_for_other_browsers(). Sadly, we lately have some bad cases where things seem similar, but instead of IE we now have FF.
One of earlier cases concerned the location.reload API. In a specific case of an iframe, the app calls this to reload the content when users change their color theme. Worked everywhere instead of FF. As seen in the docs, FF has its own forceGet parameter, not supported in the spec but mentioned in the docs. Seems that location.reload works for us in FF only when this extra argument is provided.
Another case appeared lately, unfortunately. Take the WS-Federation protocol and consider a scenario where the identity provider and the service provider are on different domains.
Signing in works correctly. The service provider redirects with wa=wsignin1.0 and the identity provider responds with the SAML token POSTed to the service provider.
Signing out is implemented using nested iframes where the identity provider points to the service provider and adds wa=wsignoutcleanup1.0 to terminate the session (drop the service provider session cookie). As you know, there's been a change lately in the way cross domain cookies are handled. To prevent the CSRF, SameSite flag was added and in the default case, the cookie falls back to SameSite=lax which prevents it from being accessed cross different domains.
There's however still a way to make cookies available in cross domain requests, you are supposed to just mark them with SameSite=none;Secure. And guess what, this works in all other browsers, except FF. It turns out, the default security settings for FF prevent all crossdomain cookies, no matter if they are marked with SameSite=none or not.
Sure, users can opt out by lowering the security level or configure exceptions for your app but this doesn't change the fact that the specific scenario mentioned above just doesn't work in the default FF setup. Other browsers have their own security settings and, at least at the very moment, you can opt in for more strict settings (and cross domain cookies don't work anymore) but this requires a change in default settings. In case of FF, it's the default setting that it's against the samesite spec.