Hi Remus - thanks for the suggestions!
As you suggested, I disabled tracking protection for the site. It made no difference.
I removed localhost:3000 (the site I’m developing on) from my history as well as auth.functionaldogdata.org (the redirect URL to auth0) and restarted. It made no difference.
I removed all storage related to auth.functionaldogdata.org and restarted, no difference. (There was cookie data, related to auth0. Nothing else). I checked for storage data related to localhost and there was none. I could not check for storage data related to localhost:3000 as I cannot access that URL currently - it just redirects to auth.functionaldogdata.org.
I’m really at a loss - the application works fine on other installations and with other browsers, so it really seems to be some data that I have faiiled to get rid of that is shared between this browser (Firefox) and this installation (localhost:3000). But I can’t figure out where it is.