HI, suggest restarting Windows 10 in Safe Mode with Network Support. *https://support.microsoft.com/en-ca/help/12376/windows-10-start-your-pc-in-safe-mode Open Task Manager and confirm there is not any Firefox Processes running and if are End them. Thenuninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files , C:\Program Files(x86) & C:\ProgramData Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles *https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer Reinstall with Current Release Firefox 61.0.0 with a Full Version Installer *https://www.mozilla.org/firefox/all/ Please let us know if this solved your issue or if need further assistance.