I guess it's because the contract is valid only for Mozilla Firefox.

cock.li resolves perfectly fine on Firefox with Cloudflare DoH.

Are you sure? It does not for me. Although I am using my distro's release of Firefox. I will be trying it on my windows pc with the official FF release later.

Sure. Maybe your ISP is blocking the domain or something.

I can access it with my ISP, I can also access it with NextDNS (over DoH) and 8.8.8.8. My friends also reproduced my results from their machines.

Can you run dig @1.1.1.1 cock.li just in case?

That command runs fine. No errors.

Dig does not throw an error when it does not get a result, instead you get an output like this:

    ; (1 server found)
    ;; global options: +cmd
    ;; Got answer:
    ;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 49352
    ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
    
    ;; OPT PSEUDOSECTION:
    ; EDNS: version: 0, flags:; udp: 1232
    ; EDE: 0 (Other): (time limit exceeded)
    ;; QUESTION SECTION:
    ;cock.li.                       IN      A
    
    ;; Query time: 115 msec
    ;; SERVER: 1.1.1.1#53(1.1.1.1) (UDP)

In contrast with a successful run like

    ; (1 server found)
    ;; global options: +cmd
    ;; Got answer:
    ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 21996
    ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
    
    ;; OPT PSEUDOSECTION:
    ; EDNS: version: 0, flags:; udp: 512
    ;; QUESTION SECTION:
    ;cock.li.                       IN      A
    
    ;; ANSWER SECTION:
    cock.li.                300     IN      A       193.239.85.202
    
    ;; Query time: 159 msec
    ;; SERVER: 8.8.8.8#53(8.8.8.8) (UDP)

Got NOERROR and got the IP of cock.li