The problem didn’t show up on my Firefox browsers in Ubuntu, Disco Dingo and Mint; however, on my main computer (‘Intel’) running WIN10 it did.
On WIN10’s Firefox I removed problem add-on Adblock Plus, then selected “Help” drop-down menu … selected “Troubleshooting Information” … then clicked on “Refresh Firefox” in upper right hand corner of that page. Reinstalled the Adblock Plus add-on and everything worked fine again.
Wonder if the Ubuntu, Disco Dingo and Mint had already had their certificate signatures updated – somehow, thru updates maybe? Ubuntu and Disco Dingo were fairly new installs. Did other Ubuntu or any Ubuntu-based Distros have the add-on problem? I know some non-Ubuntu Linux distros had the problem.
‘Ask Ubuntu’ seems to have only one issue – which then pointed to: Update Regarding Add-ons in Firefox – not much help or info at either.