It is not horizontally scalable right now, but that is not hard to implement either. You can have a look at the dogehouse project: https://github.com/benawad/dogehouse if you want to see how to horizontally scale mediasoup which is the media handler and stateful process within the servers.
I think vertical scaling will be more than enough for most of the users that are going to self-host this.
Firefox WebRTC support isn't missing anything big, but it's missing a lot of little things from the 1.0 spec that Chrome (and, increasingly, Safari) have. But, much worse from a support perspective, recent Firefox releases have been fairly likely to have WebRTC-related breaking changes and regressions.
We all try to support Firefox. But given the relative usage numbers of Firefox, Chrome/Chromium/Electron/Edgium, and Safari, it's sometimes hard to devote resources to testing and workarounds.
Anyway, this open source alternative doesn't work with the open source browser and suggests to install two proprietary ones instead.