I've been using DoH with 66.0.5 for several days at least (as the
primary browser), and even visiting the URL directly doesn't show
anything like that for me.  I wonder what I'm doing differently.  (BTW,
there's FF 67 now, too.)  Still, we/I do know that not sending the
intermediary certificate isn't a correct behavior.

Not all users will see that error because many will have the interm.
certificate already gotten from visiting others sites, but if you have
a fresh installation and visit that DoH endpoint you will get that error.

Oh, right.  Creating a fresh FF profile triggers it.  I didn't know how clients cope with the missing intermediaries (and still don't know much about it).  I'll look into why lua-http is doing this ATM.

--Vladimir