I'm setting up an Arch box and I've installed Plasma 5 as my desktop environment. However, monospace fonts are behaving very erratically:

Listed in the 'Fonts' prefpane, it displays fine.

As part of a terminal emulator, it's fine.

Firefox slants them something awful in some cases (see forum), and

will completely miss them in other cases (see post).

It's probably worthwhile to note that evaluating (set-default-font "Oxygen Mono") in emacs fails with

Invalid font name: -unknown-Oxygen-Sans-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1 ,

suggesting to me that there is some font registration issue. (Note that it's looking for Oxygen Sans…)

How can I achieve sanity and convince these fonts to behave?

Additional information from continuing research: