Firefox sends it's WebSocket upgrade request with a Connection header value of keep-alive, Upgrade. Unfortunately MessageBody.For does a strict equality check (https://github.com/aspnet/KestrelHttpServer/blob/dev/src/Microsoft.AspNetCore.Server.Kestrel/Internal/Http/MessageBody.cs#L237) which means the WebSocket request is assigned the ForContentLength version, with a content length of 0. As a result, the connection is considered terminated before it is even established and things go haywire (specifically the first attempt to Receive a message causes an exception because the underlying connection has been terminated).

We'll need to switch to either a Contains check or multiple Equals checks in order to ensure we're getting things right.

/cc @halter73