If you want to turn it off, go to about:config
and toggle browser.newtabpage.activity-stream.showSponsored
and browser.newtabpage.activity-stream.showSponsoredTopSites
to false (I'm not sure which one does it, but if you don't want sponsorships, I assume you don't want either).
I'm also unhappy about it being on by default... Unfortunately, this is probably the fault of the state of the internet, how modern browser development requires to implement unreasonably many functions, making it impossible to anyone except a corporation like Mozilla — who wants to profit off it. If there was a better alternative, I would switch in an instant, but all the other browsers that have all the functionality I need are Chromium in disguise.