Thank you Maurice for all the explanations and warnings.
Ad : So far I have not had a repeat of this browser locker yet.
Ad : especially useful for those instances when more than just the back button is blocked (as I have experienced before)
Ad : Thanks for the link; lots of useful info I was not aware of all of it yet. But there I see a problem for me: except in very serious cases I never want to clear History. I want to PRUNE History of all 'the bad actors', any malware pages that are in there. And it just does not do (in Firefox) to 'Select All Tabs' and then to 'Bookmark All Tabs' beforehand, in order to retain the tabs, as the history per tab is not saved, just the current page. And in History on the Bookmark Tool Bar the pages are retained sequentially and not grouped by Tab. I would want to snip the blocking- or malware site from the individual tab history so that if I go back it is as if the removed site has never been there. My experience with apps that retained the full history per tab was rather negative; rather unwieldy and time consuming.
Ad : I have installed the Malwarebytes Firefox Browser Guard. I hope for a positive experience.
Additionally, I would like to download and save your post as it was rather mangled by my email client; is there a way to retain it offline just as it looks online? I was not able to do it in FF or in IE.