Firefox Developer Edition Websocket Messages tab not working

Hi folks,
Has anyone tested the new websocket messages tab in 70.0b14? The tab is there, but no messages.
Here's the command sent:
"curl -i 'http://localhost:4000/socket/websocket?token=undefined&vsn=2.0.0' -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:70.0) Gecko/20100101 Firefox/70.0' -H 'Accept: /' -H 'Accept-Language: en-US,en;q=0.5' --compressed -H 'Sec-WebSocket-Version: 13' -H 'Origin: http://localhost:4000' -H 'Sec-WebSocket-Extensions: permessage-deflate' -H 'Sec-WebSocket-Key: oIt2p8nyUbkEOHoYhlngdA==' -H 'Connection: keep-alive, Upgrade' -H 'Pragma: no-cache' -H 'Cache-Control: no-cache' -H 'Upgrade: websocket'"

Here's the response:
HTTP/1.1 101 Switching Protocols
cache-control: max-age=0, private, must-revalidate
connection: Upgrade
date: Thu, 17 Oct 2019 14:31:47 GMT
sec-websocket-accept: NpK0Bknaq99vHpSzNqswmQmjz0I=
server: Cowboy
upgrade: websocket

Here's a snip of the dev tools view:

I switched to unstable branch, and it's working in 71.0b7.

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