I don't "follow" you. Once a secondary email address is added to an existing account, the user would need to make that email address the Primary email address as in cases where the former Primary email address was no longer available to that person. Such as with a change of ISP or a job change where the user had lost access to the "old" email address. And your situation wasn't thought of or it wasn't a concern that Mozilla dealt with. With so many free email services out there ....

And in my 10 plus year experience with Sync, an additional account for Sync account needs a "fresh" (unused with Mozilla previously) email address. But that was before Sync became part of the "Firefox Accounts" (FxA) scheme. And IMO, an unnecessary inconvenience for Firefox users, but brought about by the evolution of Sync over to the FxA system. Over the years I have accumulated 5 Sync / FxA accounts @ gmail ... due to issues that have arisen for me with Sync over the last 10 years due to the transition from Sync v1.0 to v1.1 then to v1.5 over the first few years of Sync's life. Sync v1.5 to the FxA system was a "piece of cake" in comparison.

ok, to make it more clear: I want to create a second, brand new, fresh account with an email address that was a secondary email address before.

but as i understood you, this seems to be impossible, so thank you very much for the very detailed answer.

weird. I wonder, how long is mozilla allowed to store unused data of mine at all?

ok, to make it more clear: I want to create a second, brand new, fresh account with an email address that was a secondary email address before. but as i understood you, this seems to be impossible, so thank you very much for the very detailed answer. weird. I wonder, how long is mozilla allowed to store unused data of mine at all?

IMO, its not a question of "allow" - have you read the ToS?
https://www.mozilla.org/en-US/about/legal/terms/firefox/

With the manner in which Sync works, there is little to no chance that anyone could ever use your data even if they found a way to "break into" the Sync servers. Not even the people who maintain the Sync servers and the Firefox Accounts servers gain access to "user data" that isn't encrypted with the "user's key" which is solely on the user's own devices and isn't sent anywhere outside of the Firefox browser.

With out one of the encryption codes (of which there are 3) your data couldn't be decrypted even my an IBM Watson level computer. It that happens to be your concern.

Did you remove this secondary email address in the other account?

• https://accounts.firefox.com/settings