Ah, sorry, I was looking at the wrong item on "Can I Use...", @font-face: font-variation-settings, rather than the "Variable Fonts" item which covers font-variation-settings outside of @font-face rules.

https://caniuse.com/#search=font-variation-settings

And I am running an ancient version of macOS, 10.12, which explains my issue, as it's only supported on macOS 10.13+ in Firefox.

I have a new (non-macOS) laptop in transit, but my current daily driver is an old Mac.

Sorry for the alarm!