When you are signed in with your Adobe CC account, open a new page for CCExpress. The landing page doesn't recognize that you already have an account - despite showing that you are signed in (upper right corner).
Is this something unique to Firefox?
or Is this the landing page for everyone?
I know that Firefox browser has had other troubles with ACS.
It should take you immediately to your Home or (better) Projects page.
Sorry for the trouble. Could you please check this in another browser, if you get the same pop up? Also, please try to log in to a new "Incognito" or "Private" browser window. Let us know if that works for you. Please return with questions or comments.