I don't care much about this sponsored tab fuss but that is some serious BS logic.
Feels like UX goes in circles. The web of the 90s was "Show as much content as possible". Google came around and it became "show only the absolutely crucial stuff". Then facebook came around and we're back to shoving as much crap down the monitors as possible.
Feels like we jump from one extreme to another all the time. And yet, the Google.com homepage remains clean and uncluttered (yes, there's more stuff than there used to, but its still 90% whitespace).
Take it this way: When you create a new folder, a file manager won't show the contents of other folders you often access just because it's "best that you see content". Or when you empty your inbox, gmail won't show some random popular emails. So on.
Showing popular sites is a good idea at its core but if you're on a fresh profile, well you're on a fresh profile. Make money off firefox of course, you need it, but "it's best you see content" is bullshit justification.