Firfox has a bunch of odd security features that I only sometimes understand.
Among them is that it won't load JS files without the `content-type` header,
and it also disables a lot of features for the `file://` scheme. Which I think
is a shame. There's probably a good reason they do this, but I'm not aware of one.
So now you can use PTTH's file server to host Jet Racing 4.