Really? Are you sure you don't have some other hosts/vpn based ad blocker? I surely don't use Firefox Preview for much, but I definitely saw an ad on Preview when I was using it yesterday. I forgot why I wasn't using it -- aside from the fact that Firefox normal is in the quick access icons -- and since my phone had just restarted I thought I'd give it a go. But I quickly saw an ad and thought "oh right" and went back to normal Firefox.