uBO just uses a browser API to resolve cname, it does not get to decide how the DNS requests are made. You will have to disable cname-uncloaking through advanced settings if you do not want this feature.

Alright I will look at this when I get the time.

If you can investigate why this does not work anymore that would save me a lot of time -- I am feeling rather burdened at the moment.

Did you restart uBO after you changed your proxy settings?

This is fixed in Firefox 80, OP is using ESR 78

https://bugzilla.mozilla.org/show_bug.cgi?id=1618271

Ok I'm hard of understanding... I will have to download 78 and test again. Also, I should fix the workaround code to execute only for versions of Firefox below 80.

I tried with 78.3.1esr and I could not reproduce. This is the proxy settings I used:

Yes, I followed this issue 6 month ago with uBO 1.25.0 and the fix (1.25.x (can't remember)) solved it for ESR 68.x back then.

I tested with three different profiles yesterday and same issue on all of them (they have different privacy/security settings).

FF ESR got upgraded to 78.x last weekend or so, and I noticed this issue was back as of yesterday. It is very strange that it got introduced again. But yes. I will disable cname for now and wait for FF 80+.

How do your proxy settings differ from those in my screenshot?

They're the same. Except I have mine filled with localhost and ports of course, as seen in picture 3.

I cant repro it, set

cnameUncloak true
cnameUncloakProxied true

Set DNS to be resolved through proxy (shadowsocks). Tested on
https://dnsleaktest.com/
and
https://browserleaks.com/dns
No leakage.

Firefox 81

@Kein Well of course, the purpose of cnameUncloakProxied when set to true is to bypass the built-in protection. Leave it to false (default) if you do not want uBO to uncloak when a proxy is detected.

Does not make any difference for the repro case. In fact, any cname-related directives do not affect repro case and it always fails.

I'm not asking you to do anything, I just pointed out I cant reproduce the case OP mentioned either on FF81, that all.

Duh me, I read your "I can't repro" as "I can repro". Sorry for the confusion.

Well, I am utterly confused now.. I have two other profiles (same Firefox 78.3.0 ESR) that uses the ghacks pants user.js stuff. And those do not leak DNS. But when I look at the stuff in about:config regarding proxy settings they are exactly the same as the non ghacks profile that do leak DNS and made me report this in the first place.. I do not understand it. I will wait for FF 80+ and see if they really fixed it or nah. But it seems so since people can't reproduce it. As for this issue I am fine about it being closed as can't reproduce and firefox bug. TIA for your time. And thank you for uBO.

I cant repro it, set

cnameUncloak true
cnameUncloakProxied true

Set DNS to be resolved through proxy (shadowsocks). Tested on
https://dnsleaktest.com/
and
https://browserleaks.com/dns
No leakage.

Firefox 81

Do never forget ISP(ISS) and OS changeable!
They can send fake (request and response) data