Firefox usually uses a temp file like logins.json.tmp to ensure that the file can be stored correctly in the profile folder on the hard drive. Once this file has been created correctly then Firefox will remove the original file and rename the temp file. If you keep the temp file then this can indicate that the original file is locked by another application or otherwise can't be removed and replaced by a new file. Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems. *http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/ *http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10