Ccleaner Free (WIN 8.1) now 3 hours cleaning on internet mozilla internet cache. Used this program for years - never had this problem. What to do?
perhaps this help:
save your ff-profil or only your bookmarks on another place (usb or other partition), deinstall ff and delete all mozilla/firefox-folders in your profil in windows. reinstall ff and insert your copy of your bookmarks back
english:
https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
german:
https://support.mozilla.org/de/kb/firefox-profile-sichern-und-wiederherstellen
Make sure firefox.exe isn't in the list of running processes in Task Manager (CTRL+ALT+DEL).