They have a contract with CloudFlare with stronger privacy protections for FF users. This goes beyond the normal CloudFlare privacy policy. https://developers.cloudflare.com/1.1.1.1/commitment-to-priv... They're not just picking providers at random.
In general I agree that using other DNS providers besides the ISPs has benefits. But not sure I think it should funnel to a single, preferred company. Nor do I think that they should collect anything at all, reduced compared to their original or not. I think some transparency is deserved here about the contract if/when a regular FF release is shipped encouraging users to use Cloudflare, specifically around the motives of wanting this data. Granted, I am a bit more paranoid than most.
If they intended to ship with DoH default-enabled, I'd hope Mozilla would not go with any single provider as a default.