If you insist on using a third party resolver for name resolution they will have knowledge of your queries no matter what the protocol. Doing it over tcp and http is not any better, or worse, than doing it over udp. This is something you have to opt in to.
Yes, the difference is that 0 is the default value right now, so if you set it in about:config all Gecko remembers is "it's the default" and stores nothing in user.js; if the default then changes the value changes.
5 is not the default, so if you set it it will get stored in user.js and then even if the default changes the value will remain 5.
If you insist on using a third party resolver for name resolution they will have knowledge of your queries no matter what the protocol. Doing it over tcp and http is not any better, or worse, than doing it over udp. This is something you have to opt in to.