> 1. `ui-monospace` - remains unsupported by Firefox which is lame (would also render 'Consolas').

Nothing but Apple operating systems even has a concept matching ui-monospace/ui-sans-serif/ui-serif/ui-rounded. And no one but Apple has implemented them in their browsers. I don’t believe they should be included in any standard: the very concept is not, in practice, cross-platform.

Even system-ui is super dodgy to use. In practice people have been using it as a proxy for a possibly-prettier sans-serif, but that’s just not what it is. The vast majority of its usage is inappropriate.