AFAIK, the default nowadays(91.0a1) is to deliver 8 tiles as default for new profiles, hence I think there is nothing actionable here. Marking as WFM, please reopen if I misunderstood the intent here.
Bug 1437494 Comment 5 Edit History
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AFAIK, the default nowadays(91.0a1) is to deliver 8 tiles placeholders + 1 row topsides enabled as default for new profiles with the default topsites coming from RemoteSettings top-sites configuration since Fx87, hence I think there is nothing actionable here. Marking as WFM, please reopen if I misunderstood the intent here.