See:

• https://support.mozilla.org/kb/what-is-persona-and-how-does-it-work

If Persona is supported for all Mozilla domains then you will be able to use that to log in to all supported domains by using the same Persona account.
Currently you need to register separately for most sites, so you need to register to the SUMO site (support.mozilla.org) and set up a Sync account (services.mozilla.com) to be able to use Sync.
I don't know if it will ever be possible to use a Persona account for Sync.

• Bug 714934 - add support for browserid/persona authentication

Persona was going to be introduced as the sign in method for this forum last Thursday, but although the software is in place and ready to go the idea is shelved for now whilst Firefox OS sign in is sorted out and made compatible with Persona.

• https://support.mozilla.org/en-US/forums/contributors/709638?last=54479#post-54475

Sync needs an email address but is not using Persona

• Firefox Sync data is secure - Find out more
• Where can I find the code to add a device to Firefox Sync?

IIRC development of Sync has ceased and there is a proposal for a new replacement service where it is hoped that syncing with server copies would be routine instead of the current intention of syncing with second devices.

Cor-el, I am confused by your statement, "I don't know if it will ever be possible to use a Persona account for Sync." I don't really understand the difference between the two. Are there different kinds of "sync" or something?...

Also, when you say, "Currently you need to register separately for most sites, so you need to register to the SUMO site (support.mozilla.org) and set up a Sync account (services.mozilla.com) to be able to use Sync." Do you mean that once you register separately for sites, you can then login to any of them via one password? EG. if i register for site A with password AAAAAA, then B with BBBBBB, then C with CCCCCC. Then I can login to any of them with one persona password, PERSONA? I don't know, its hard for me to grasp the concept, so if you wouldn't mind sharing a concrete example that a non-tekkie like me can understand...? Thanks lots!

If you set up Sync then you currently need to enter a user name and password .

If you would use Persona to log in then you would need to be redirected to the login.persona.org website and log in via that page and that is currently not implemented.

Websites like MDN and Bugzilla can use Persona to login, but this is not possible on the SUMO forum, so you need a separate password for SUMO.
If a server supports a login via Persona that you will see that on the login page via a special Persona "Sign in" icon.

• https://developer.mozilla.org/
• https://bugzilla.mozilla.org/

john99, what do you mean by, "syncing with server copies"? and how would that work?

also, is syncing currently between devices, as opposed to Persona, which is between FF sites?(EG. support.mozilla.org, or account services.mozilla.com.) ? Thanks!

Sync is currently to sync personal data to other devices.
That can be other profiles and/or Firefox versions on the same computer or other computers and mobile devices.
Some like to use Sync as a backup system for only one computer and that is not implemented reliably and can fail at any time especially if you reinstall Firefox and want to retrieve data stored on the Sync server.
The services.mozilla.com web page is where you can check your Sync account.
This server doesn't support sign in via Persona (no Persona icon on the login page) and requires its own password.

Persona is only a special way to authenticate yourself to Mozilla servers independent of the web page (Mozilla server or sub-domain).
This requires JavaScript to pass this data to and forth between the servers, so the server needs to support this way of authenticating.

Awesome, I get it! :) thanks cor-el!