Please explain what you mean with "contacts".

Your email address is your account name / username. If you need to changes the email address for your Sync account, you need to create a new account.

Set up Sync initially. https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer

Add additional "devices" / PC's. https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer#w_connect-additional-devices-to-sync

contacts are the people with whom we talk. "invite a friend to join you" is on the page , after start a conversation. I try to compare this to Skype that I used to have and has been lost/

I assume that you mean Hello and not Sync?

• https://www.mozilla.org/en-US/firefox/hello/
• https://support.mozilla.org/kb/create-and-manage-your-contacts-list-firefox-hello