I think the fellow you quote here didn't quite understand the "right source", because he/she contradicts it. The preference doesn't exist by default, but the default action of Firefox is to create the screenshots; and the default value for the preference is 'false'. At least, that's what MDN says--not disabled by default.
Could it be the version of Firefox you're using or a conflict with an add-on?
OK... never mind--it's working for you now (and you had 3 edits while I completed my interrupted post
[Edit:] Of course, it's possible that the MDN page was incorrect and was updated due to this fellow's post bringing attention to the fact? ...check out those dates. --
"Face piles of trials with smiles; it riles them to believe that you perceive the web they weave."