I believe what you saw was a promotion of https://addons.mozilla.org/en-US/firefox/addon/facebook-cont...
That extension is described as:
"Prevent Facebook from tracking you around the web. The Facebook Container extension for Firefox helps you take control and isolate your web activity from Facebook."
Today, in fact, there is a huge pseudo-advertisiment (at least in fr_FR) about "how I should not feel guilty about browsing Facebook [through Firefox]" , which almost made me puke.
As a person who doesn't care about Facebook whatsoever, I didn't get the impression that FB was enemy #1, but rather I assumed FB was paying them money like Google is...
I can actually see FB being happy to just give money away to Mozilla, just to influence them away from Google.