As of Feb 2023, itch.io supports SharedArrayBuffer on Firefox, so this document should be unnecessary.
Itch.io has recently implemented the ability to use SharedArrayBuffer on its site, which means you can export games with Threads enabled for HTML5. You can learn more about this here. This is great for Godot, as it helps to improve/fix the sound laggyness that sometimes occurs in web builds. It is also now required for Godot 4.