I would find it useful to be able to have multiple Firefox accounts in order to have more control over what gets synced (and other functions). Currently, I have one Firefox account for both my wife and myself. This makes it possible to sync bookmarks and extensions, making our various systems easier to maintain. Also, it would allow more flexibility when syncing with mobile versions of Firefox, since extensions cannot be synced beween desktop and mobile versions of Firefox.
Another thing is that I have one Firefox account for both myself and my wife because of the syncing that I want. The downside is that when I login to a forum like this one, I don't have my unique name.
Roger Fajman