This site uses your Firefox Account as your support site account. You can have multiple Firefox Accounts: one per email address. You can sign out and use your other account if you prefer.

If you have never set up a Firefox Account before:

It might be faster to create a new one with your preferred address rather than try to change the existing one. You then can use the new account for Sync and other services.

If you already have a Firefox Account set up for Sync:

I'm not sure why this site did not detect that. Is your existing account showing on the main menu toward the top (this is the menu at the right end of the main toolbar)? You can sign out on this site and then sign in with your regular Firefox Account credentials.

Will I lose data, bookmarks, etc? Will the profile now on my desktop become worthless in terms of restoring Firefox if it crashes?

You can lose data if you try to change your Firefox Account password without knowing the current password or entering a previously saved recovery key. More info: Change your Mozilla account password.

I'm not sure of the implications of switching accounts. However, you can first back up all your locally saved Firefox data as a "just in case" using the steps in the following article: Back up and restore information in Firefox profiles.

Thank you, jscher. I deleted one account and set up a new one--I think; I find this one too is linked to an old Support account (I've been using Ffox for eons). Bewildering, frankly, but as long as it works I won't argue.