Do you use a password on your computer? Does anyone else use it? Programs do not disappear on their own, unless they are programmed to do so. Installers, for example. You may have ad / mal-ware. Further information can be found in the [[Troubleshoot Firefox issues caused by malware]] article. Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.