Doesn't seem to matter though.
I never claimed (or implied) that staying at NSS 3.44 Beta (instead or reverting to 3.43 Stable) would have allowed for the UXP browsers to successfully connect to the linked test server, i.e. https://tls13.1d.pw
As I was already talking about the NSS library, I simply found an opportunity to mention the version downgrade I had observed; just that ...
Today I borrowed briefly sister's Windows 7 SP1 64-bit laptop, where I updated stable Firefox (Quantum) to latest release 67.0 and Firefox ESR (Quantum) to latest release 60.7.0; to my great astonishment, both these Firefox versions connect successfully to the TLS 1.3 test server being discussed in this very thread; I dug around and found v60.7.0 comes with NSS 3.36.7, while v67.0 comes with NSS 3.43, the same one to be found inside the latest UXP forks released by Roy; so, despite my initial assessment, the "bug" observed here ("SSL_ERROR_RX_MALFORMED_SERVER_HELLO" error code when trying to connect to test server with latest New Moon 28/Serpent 52.9.0) doesn't appear to be only dependent on used NSS version and has been already fixed in latest Firefox Quantum releases!
I persevered with my searches and, in the end, I stumbled upon a similar issue that was also affecting the Waterfox fork:
https://github.com/MrAlex94/Waterfox/issues/783#issuecomment-458907249
So it looks as though the proper fix for this issue lies inside Bugzilla bug #1430268 fixed in mercurial commit:
https://hg.mozilla.org/mozilla-central/rev/fafb731
@roytam1: Do you think you can somehow backport bug #1430268
to the UXP platform and thus apply it to both NM28/St52? Is a test-build even possible? If yes, then the OP (@Bersaglio) won't have to use a Chinese browser to connect with...
OT: I don't mistrust myself the Chinese browsers any more than I do American ones like Google Chrome or Russian ones like Yandex Browser; all government security agencies are pretty much the same in my eyes... ; in this day and age, there's no such thing as privacy if you decide to connect to the live web...