If you do not remember the password for the Firefox Account then you will have to reset this password. This will remove all data stored on the Sync server, but data stored on connected devices shouldn't be affected. You can do a backup to be sure. *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles If you use Sync then best is to reconnect one device at the time and wait for the initial Sync to finish before connecting another device.