The only way Cloudflare itself can cause users a privacy issue through the use of its DNS service is by lying to them. They have already agreed not to sell or grant the data to any third party or use it for advertising, etc.
The other concern nullc has is that hackers will infiltrate Cloudflare because it will now contain data on all US Firefox users' DNS queries unless those users change default settings. OK. I guess. However, I will say that I trust Cloudflare a whole heckuva lot more than I trust Comcast, Charter, AT&T, Verizon, etc. to be practicing good security. There are also probably more people on the mega ISPs than there are Firefox users, and many DNS queries these days originate from platforms that aren't even PCs (such as home theater streaming sticks) which will still be pinging default DHCP-received DNS providers. If anything, I see this decentralizing DNS request data.