hello, I do not know exactly where to put this so I put it here … sorry if not ok … ( I am french speaking by the way ). I really would appreciate a add-on in Firefox ( that I always use as my favorite browser ) that permits me to MARK the last word I was reading in a long text when the phone did ring … now I always have to do a bit of research to refind the place where I stopped reading for the disturbing phone call … some kind of big marker that I can click ( left click ) and puts a big red marker ??? If there is a answer please how will I be signaled it ? my email = geomar@scarlet.be. Thanks a lot for attention. 
Hello,
I think you might have more luck posting this here instead: https://discourse.mozilla.org/c/firefox-development
This forum is for DevTools specifically.
But, what I would advise is first of all the Firefox reader mode (which you can activate form the URL bar). It doesn’t have this feature you’re talking about, but it’s a very distraction-free reading environment that would make it easier to find where you left off.
The other, very simple, trick I use is selecting some text with the mouse, to remember where I was. Very quick to do, and acts as the marker you were mentioning in my opinion.
Now, if these don’t work for you, creating an extension for this would probably be quite simple.
hello hello ! thanks a lot for your quick response !!! ok I will look for it but yes that seems to be a good thing to do as you say : just select as I do for copying … why did I not think of that hahaha … I will try ! thanks a lot and have a nice day !!!