Firefox can not cause those files to disappear as it is not hooked to those places in any way, you must have had another update waiting, or a glitch when restarted or a MS Update which can check if did in Settings --> Update & Security --> View Installed Update History. I would suggest you do a system Restore and see if can get your files back by using Windows 10 Search and type System Restore. At the top of the List will be Create a Restore Point. Click on that then a small box thing opens and Click on System Restore. If need further help with Windows please see : *https://www.tenforums.com/ *https://www.bleepingcomputer.com/forums/f/229/windows-10-support/