Doesn’t seem to be working on latest Firefox ESR (or, according to the error message, on the web server backend they use)?:
Error
The following features required to run Godot projects on the Web are missing:
Cross Origin Isolation - Check web server configuration (send correct headers)
SharedArrayBuffer - Check web server configuration (send correct headers)
From the perspective of looking at the internet of today, the future is 1996.
Change my mind.