Niagara, but I've also been trying out Octopi every now and then, it's still in beta and there are certain features missing but it feels like it has potential to fully replace Nova.
I have seen the "thought provoking stories" on a new FF install I did a couple of days ago, it seems to be the replacement for Pocket stories since that's been removed. You can still remove it in about:preferences#home and uncheck "Recommended stories".
However I have never seen the ad below it either. Not even sure where that's coming from, there is a ... button on the top right of it, I would be tempted to see if that gives any hint where it's coming from.
Niagara, but I've also been trying out Octopi every now and then, it's still in beta and there are certain features missing but it feels like it has potential to fully replace Nova.