Thanks to Bowdon (and friend) for listing the method of enabling "studies" in Firefox, found that it was not enabled on my main, Win 10 desktop PC and add-ons had been disabled, however, on my backup, dual boot Win 10/Linux Mint PC and my Win 10 laptop PC Firefox "studies" was already enabled, all running the latest version of Firefox.
After enabling "studies" my add-ons were re-activated (un-blocked) within 15 minutes but had to be manually enabled again.