Go ahead and install Firefox 66 again. I recommend doing it this way:

Clean Reinstall

This takes about 5 minutes. It's not about your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. It's not essential to uninstall Firefox, but you can if you like, saying No to any request about removing personal data.

(A) Download a fresh installer for Firefox to a convenient location, either:

• Standard Release (Firefox 66): https://www.mozilla.org/firefox/all/ (recommended)
• Extended Support Release of Firefox 60: https://www.mozilla.org/firefox/organizations/all/

(B) Exit out of Firefox (if applicable).

If you use Microsoft Office, please change your default browser to Internet Explorer temporarily before the next step.

(C) Using Windows Explorer/My Computer (hold down the Windows key and press E to launch it), right-click > rename the program folder as follows (you might have one or both):

C:\Program Files (x86)\Mozilla Firefox =to=> C:\Program Files (x86)\OldFirefox

C:\Program Files\Mozilla Firefox =to=> C:\Program Files\OldFirefox

(D) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Any improvement at your next startup?

By the way, if you want a pre-release version to run alongside the regular release, the "Developer Edition" might be a better choice because it recently was rejiggered to keep its data separate.

https://www.mozilla.org/firefox/developer/all/

Thank you for your assistance on the original issue.

jscher2000 said

By the way, if you want a pre-release version to run alongside the regular release, the "Developer Edition" might be a better choice because it recently was rejiggered to keep its data separate. https://www.mozilla.org/firefox/developer/all/

I've been using unstable editions of Firefox before but I've never seen anything like it! I downloaded the official installer for Firefox Nightly. I launched it and couldn't believe my eyes! It started doing its thing without a single input from me. Also without an input it launched the Nightly... with my standard non-beta profile. When I issued firefox.exe -P to launch the profile selector the damn thing had the standard "Start Firefox" button replaced with "Start Firefox Nightly"! Guess where it has nested. In a continuously swelling C:\Program Files\Mozilla Firefox\tobedeleted\ folder. I'm done with exclamation marks at this point because they don't even do the whole hiatus justice.

Not long ago I was using Firefox Nightly to easily get to run two instances of Firefox (non-beta alongside the Nightly). Now I got an unattended malware that replaced my non-beta Firefox installation. Simply astounding.

Out of curiosity I consulted the kb zine article as well as other support entries on the subject. Not a word about the changes.