Have latest version of Firefox...
Just installed Adblock Plus(12-31-17) ...
Tested it on Youtube--The main reason I got it
was to prevent preroll ads or any ads that interfere
with watching the chosen video...

It failed the test! I was so disappointed.

I immediately removed it.

Searched for alternative, found and installed AdBlocker Ultimate...

Tested it right away on Youtube...

Hooray! Worked like a charm!

Hope it will continue to do so, as I assume Adblock Plus worked well for some time.