Although Joe Average won't know how to, most/all OSes let you pretty easily change your DNS server, you don't have to use your ISP's. But Firefox's UI to _not_ use Cloudflare is _way_ less straightforward.
Most/all routers let you change the DNS that's handed out, too, even the all-in-ones given out by the major ISPs still let you change the default DNS for the entire network.
But also somewhat common[1] is the router handing out itself as the DNS server, which is really important if you want local domains to resolve correctly. Firefox skipping straight to 1.1.1.1 means it won't be able to resolve my local network servers via name, which is stupid.
1: Maybe not common/used in home use sure, but definitely common in anything run by an IT staff.
Even in Europe big telcos like Telenor have adtech holdings.
It doesn’t mean you can’t trust your ISP but certainly there are red flags.