The support.mozilla.org site uses Firefox Accounts to post with for a while now as it used to be an account system that was only for this site. So you need to go to https://accounts.firefox.com/settings if you want to change things like the Account picture. Mozilla does have @mozilla.com @mozilla.org email but it is only for their own use and given to people who work for Mozilla in a capacity that requires it. Mozilla does not have any email service for the general public.