The article is right to be fearful that FF is pondering a default change, but until that is even on the table, I'm not worried. Now if they wanted to make it really really easy for regular users to change from your default ISP DNS to CloudFlare, I'd actually be OK with that, but I'd expect it to be implemented like search engine providers where anyone could just as easily be the DNS provider chosen (ideally without any CloudFlare favoritism). And it would be clear who your DNS provider is maybe via an icon (if there is real estate for it).
0 - https://blog.nightly.mozilla.org/2018/06/01/improving-dns-pr...
1 - https://blog.nightly.mozilla.org/2018/08/28/firefox-nightly-...