storage/permanent/chrome/idb/ strange files

Hello, recently I found these files: C:\Documents and Settings\[name]\Application Data\Mozilla/Firefox/[number].default/storage/permanent/chrome C:\Documents and Settings\[name]\Application Data\Mozilla/Firefox/[number].default/storage/permanent/chrome/.metadata C:\Documents and Settings\[name]\Application Data\Mozilla/Firefox/[number].default/storage/permanent/chrome/idb C:\Documents and Settings\[name]\Application Data\Mozilla/Firefox/[number].default/storage/permanent/chrome/idb/2918063365piupsah.files C:\Documents and Settings\[name]\Application Data\Mozilla/Firefox/[number].default/storage/permanent/chrome/idb/2918063365piupsah.sqlite I can delete them, but they reappear with every Mozilla restart. also, in the folder ...\storage\permanent\chrome\idb temporarily appearing (for couple of seconds) two files - 2918063365piupsah.sqlite-shm (32 kb) and 2918063365piupsah.sqlite-wal (0 kb) Are those files (2918063365piupsah.sqlite / 2918063365piupsah.sqlite-shm / 2918063365piupsah.sqlite-wal) SAFE (that's the most important question)? Really weird names and location. And what are those? What makes them appear? [Firefox 44.0.2. Windows XP. Three antiviruses didn't show anything suspicious]