Mozilla’s Firefox Hardware Report has the latest desktop OS share across all Firefox channels:

  51% Windows 10
  27% Windows 7
   6% Windows 8
   6% macOS
  11% Other (Linux, BSDs, etc)

https://data.firefox.com/dashboard/hardware

I think the Firefox Nightly, Developer Edition, and Beta channels have a slightly higher share of macOS and Linux users, but not much. There are a lot of Windows users out there; you just don't see them at JS conferences. :)