Please consider turning on send info to Mozilla from with in Firefox before posting a question. This helps us help you. I have no info from your system that would help see things that cause issues. Hi, you should try to figure out what that error message was from Avast as that maybe the issue which you can check on the Avast Forum pages ; https://forum.avast.com/ and see if others are having issues after they updated and Microsoft Update and Flash and Firefox updated yesterday. Please grab a copy of your Profile as some of it maybe useful later : https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data Please : 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 59.0 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.