Firefox apparently is trying to jump the shark with even more pocket-esque behavior. This is why I install Pale Moon (and its Linux page) on my new builds instead of Firefox.
I normally schedule my posts, in case you couldn’t tell, for the morning of every fourth day. But this post is going out immediately.
Disable Firefox’s “Contextual Feature Recommender” with this entry in prefs.js:
user_pref("browser.newtabpage.activity-stream.asrouter.userprefs.cfr", false);
You can extrapolate the about:config option.
Other places on the Internet show you how to do it through the gui if you prefer.
References
My own research (diff prefs.js prefs.js.old)
Knowledge Base 