Secondary email address.
I decided to add a secondary email address to my Firefox-account, and then swap it with my primary address. After that I wanted to delete the secondary address (that I´m only supposed to use at work), in order to add a new email-address. However, by clicking "save" a red alert message was saying "Email already exists". This confuses me, as it is not shown in my settings! I´ve checked the email-address (which I use since the late 90s) and it works fine...and I might have tried to do the same operation several years ago.