If it is not uninstall 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 61.0.1 with a Full Version Installer *https://www.mozilla.org/firefox/all/ If you have backed up and saved your Profile Folder and Bookmarks : *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data Grab a copy. Now you got all you want to save as this will if works properly delete it all and is not what we recommend unless extreme circumstances : https://www.revouninstaller.com/revo_uninstaller_free_download.html Then use the URL for the Full Installer above. Please let us know if this solved your issue or if need further assistance.

This is a problem with 7-zip that is used to compress the Firefox installer and that isn't able to create a folder in the temp directory to extract the archive content. Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems. *http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/ *http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10