The content filter APIs most definitely support it[0][1]. uMatrix puts websocket requests in the "other" category on FF.

uBlock is less powerful than umatrix in some areas, so that might be an issue with the addon, not the browser.

[0] https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/... [1] https://developer.mozilla.org/en-US/Add-ons/WebExtensions/AP...

uBlock Origin sees, reports, and can block websocket connection attempts on Firefox. The onus is on the parent comment to make the case with something more substantial than "No it doesn't". Maybe he is just referring to the plain "uBlock", which is no longer maintained.