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I know there was an extension called 'websocket-monitor', but it's no longer available.
It seems that nobody can view websocket frames by firefox.
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The WebSocket inspector will be released in Firefox 71, but is ready for you to use in Firefox Developer Edition now.
That is currently not possible with the latest version of Firefox, but it has been logged and there is an open thread in Bugzilla regarding it here.
Update September 2019: According to this tweet from Mozilla, the WebSocket inspection is now working natively on the Firefox Developer edition.
Update October 2019: According to this extensive blog post from Mozilla, the feature will be released in Firefox 71. Until that, you can use Firefox Developer edition.
Update December 2019: Firefox 71 got released today and includes WebSocket inspection according to the release notes.
You can download the Firefox Developer edition from here.
Update October 2020: Sometime between Firefox 71 and Firefox 79 the "Messages" tab was removed and instead the WebSocket messages inspection was moved to the standard "Response" tab.
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It's now possible to view/sniff WebSocket messages/frames/packets with the Nightly build of Firefox. I have never used the Firefox Nightly build before, but it seems to be working okay.
Here's a screenshot: 
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