Yes, this appears to be correct - the Firefox API supports filtering out of the box. If you check the readme on the WebSocket companion, it specifically mentions Chromium based browsers, assumedly indicating it's intended for them.

There is also a note on the normal uBlock repo that says that Firefox has an extra feature with a link to inline script tag filtering, which says it does not work on Chromium based brownsers: https://github.com/gorhill/uBlock/wiki/Inline-script-tag-fil...