my firefox, which have used for years, does not now open as a browser. it looks like a blank screen beneath the standard 'file edit view history' bar

If reinstalling the browser didn't help, I would suggest a fresh profile next: # Create a new profile: https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles#w_creating-a-profile. # Run profile manager, select new profile: https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles#w_starting-the-profile-manager. If the issue is solved by a new profile, you can try Refreshing your old profile: https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings. The process is quite simple and does make backups, but I would suggest making manual backups as well, just in case: https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles#w_backing-up-your-profile. This sounds like a bad theme or profile/settings corruption, but could also be malware messing with the browser. Have you scanned your machine just in case?