I use Firefox last Developens version. Before this I use addon https://addons.mozilla.org/en-US/firefox/addon/websocket-monitor/ but now this module don’t supported by new version.
I don’t know, but today, a lot of people using websocket (or socketio) and this feature need to be as air (at least for me).
I am planning to rewrite the existing extension and make it compatible with Firefox 57+
Here is a bugzilla bug you can follow:
https://bugzilla.mozilla.org/show_bug.cgi?id=885508
And here is the source repo:
(new extension will be here too)
Here is some prototyping (from last year)
Honza
Thank you for fast answer and your work!
Hey, basically, this is the only issue which prevents me from switching to Firefox from Chrome. I really need websocket debugging 
Same here, need web-socket debugging!
Just to update this thread and people watching: Websocket inspection is ready for testing in Nightly and work is still being tracked in bug 885508, in case you want to file bug or feature requests.