Imo open sourcing is not necessary, as that is unrealistic depending on the licensing of libraries etc. Just distributing the server and making it possible to selfhost would be enough.
Or, built local network play in to the game itself (although this requires most of the work necessary for creating a selfhostable server anyway).
The problem is that if Firefox does not support features like WebGPU, people will switch to Chrome once they notice web sites don't work correctly.