I'm on Windows 11 and only Normal 400 is working in Firefox 100 Stable, Firefox 101.0b3 Developer Edition, Tor 11.0.10 (FF 91.8.0esr), and Tor 11.0.11 (FF 91.9.0esr)
It does work in both Chrome and Edge, so I don't think it's a general OS issue
Chrome renders text differently to Firefox. Chrome uses Skia: https://www.chromium.org/developers/design-documents/graphic...
Firefox uses the Windows DirectWrite APIs. This means that Firefox leans into the support that Windows has for variable fonts, and that was dicey until a few releases ago.
I’m on Ubuntu 21.10 and I know firefox supports at least the weight, slant, condense and italic axis (although there is some weird interoperability issues between the three browsers on how the italic axis)
caniuse reports that Firefox has supported variable fonts since 2018: https://caniuse.com/variable-fonts
EDIT: Firefox's variable fonts support depends on OS support, and so requires at least Windows 10 or macOS 10.13 on the desktop. Apparently, variable fonts support on Linux is "complicated". https://v-fonts.com/support