Sorry, you can't recover a Firefox Account without having access to the email that was sent; there was a code in the hyperlink.

As far as someone else being able to access your data via Sync by using that hyperlink; Sync would detect the different in devices and IP addresses; plus there is a time element involved. Not sure of the exact "time" but 24 hours later and the hyperlink won't work, based upon previous questions I have read here.

Your data sits on the Sync server as an encrypted "blob". The "key" to decrypt your data is only on your devices that you had connected to that Sync account; a different "key" just won't work. And the encryption "key" isn't even sent to the Sync server - only your Firefox installation have it.

Thank you for your answer. However, what stops the new owner of the domain from creating a mailbox with the same username as in my old E-Mail address and then using the password reset function to access my Firefox Sync account and therefore get access to my data?

There is a security feature on Firefox Sync, if the password is reset the data that is stored on the server is erased.

Glad to hear that, thanks.