Let me just offer a suggestion. I don't really use Firefox sync, but try going to Menu>Preferences>Sync>Manage and see what you get.

Hope this helps!

Maybe the list in History > "Tabs from other devices" will reveal this.

rnoor said

Is there a way to see a list of all the devices that are connected to my sync account?

Here you go:

about:sync-tabs

This assumes that a user has turned on tab-syncing.

I'm sure there's some record somewhere that shows who logged in from where to sync to the account. Each firefox instance/device has been a specific name and has an IP address that is probably/hopefully communicated when the sync occurs. Would it be very difficult to show that log to an authenticated user?