I had a hard time registering my "new sync" account because it never showed the email and password fields. At some point I found out that allowing cookies to be set works around the problem. Now I noticed that when clicking "manage accounts" I run into the same problem. This time I could just allow cookies for accounts.firefox.com which should IMHO be special cased when cookies are not allowed or at least give the user some help understanding which cookies are required.

Seems like the about:accounts jelly just shouldn't depend on cookies this strictly?

The hosted code uses localStorage to store information locally across state/page transitions. We should definitely not show a blank page here. We're working on it: https://github.com/mozilla/fxa-content-server/issues/738