Assuming that you are referring to the Sync service, '''"sign in after some years"''' is disconcerting. Sync isn't intended to be used for long term storage of Firefox user data for extended periods of time, its purpose is to synchronize Firefox data between multiple connected devices each time Firefox is opened. Depending upon how long "some years" was you data may not be on the the Sync server any longer; and if "some years" dates back into 2014, you may have been using an older version 1.1 Sync rather than the current Sync 1.5 and that old Sync 1.1 server is gone now. There were major updates made to Sync services with Firefox 29 and for quite awhile both servers were being used; Firefox 28 and earlier on Sync 1.1, and Firefox 29 and later on Sync 1.5.