Thanks KarDip. A very helpful well-written set of suggestions.
In my case, I encountered this PR_CONNECT_RESET_ERROR when using Firefox fairly tightly locked down, ( I already use DNS over HTTPS and specifically defined DNS servers). The problem occurred after a Firefox update. My problem turned out to be "Kaspersky intercepts and scans encrypted connections." as turning off "Encrypted connection scanning" in Kaspersky. At least this appears to solve the problem.
I'm guessing Firefox slightly changed the way DNS over HTTPS is done, and it didn't match Kaspersky's checking.
