[stuuuuuuuuu]

Traffic between you and the public DNS servers isn't encrypted, so your ISP can still read it.

(I suppose this is one of the problems that DNS-over-HTTPS is designed to fix.)

[Kelamir]

Thank you for the answer, stuuuuuuuuu! I'll look into it.

...

DNS-over-HTTPS can be enabled in Firefox via Network settings, turns out.