Firefox - Cannot Install Adblock Plus In Windows 7 "standard User" Account

System: Windows 7 Home Premium 64 bit
Firefox 66.0.3

I installed Firefox and wanted to add Adblock Plus. I was able to install Adblock Plus in an "Administrator" account but could not install it in a "Standard User" account. Here is what I did in the standard user account.

(1) Launch firefox (not in private mode)
(2) Go to Add Ons manager
(3) In "Find more extensions" search box enter "Adblock Plus"
(4) In search results, click on Adblock Plus
(5) In next screen, click on "Add to Firefox"
(6) Immediately get an error message "Download failed. Please check your connection"
Which makes no sense - my Internet connection is working fine.

So go to site https://adblockplus.org/
(1) Click on "Agree and Install for Firefox"
(2) Click on "Allow" in next screen
(3) Receive error message "The add-on downloaded from this site could not be installed because it appears to be corrupt"

None of the above makes sense. I have installed Firefox with Adblock Plus in the past without problems. Why does this not work now?

 

See this thread.
https://forums.majorgeeks.com/threads/firefox-just-disabled-ublock-origin.320315/