Connection warmup wasn't working properly - only works when trying to
establish WebSocket first, then workaround kicks in. Call WebSocket
endpoint without auth info expecting 401 error, but enough to initiate
cache for subsequent WebSocket functionality.
Scope this **dirty** trick to Firefox users only. Haven't figured out
how to detect proxy from JS code simply.
Tested in staging and works on our constrained WiFi.
be63993ba2