Congratulations NextDNS! You've been relentlessly executing on every front [0] with super-novel solutions [1] that a few, if any, incumbents have matched [2].
That said, I'm surprised Mozilla doesn't look at the uptime metrics before partnering with TRRs. I've been using NextDNS ever since it was announced here [3] and have been subject to a fair share of "outages" including once when everyone at home thought the internet was down...but couldn't remedy it [4].
Cloudflare's data-plane availability with 1.1.1.1 is a tall-order to match for anyone that's not Google or AWS [5]. The NextDNS founders built DailyMotion, so I'm guessing they know a thing or two about high availability and hopefully fix whatever they need to before they GA with Firefox TRR.
I must point out that Adguard DNS [6] is a viable non-configurable free alternative, which is what I now recommend to folks not savvy/bothered enough to configure NextDNS. It would be wonderful to see them added to TRR.
You now need to read the next step in the dance, which the NextDNS people explained back in 2019 as "Security implications of CNAME Cloaking".
* https://medium.com/nextdns/cname-cloaking-the-dangerous-disg... (https://news.ycombinator.com/item?id=21604825)