The backend was called BrowserID, and it was supposed to integrate with your browser and be supported by email providers not be some Mozilla service. It's only

The backend was called BrowserID, and it was supposed to integrate with your browser and be supported by email providers not be some Mozilla service. It's only a service because they didn't get around to implementing the actual BrowserID design, so they run a service for now, and then they halted development before they even tried to build the actual design!