I’m not sure it’s related to MediaSoup at all but is it mandatory to have an SVC-supported browser on the viewer side as well to be able to use SVC?
If you mean VP9, Chrome can generate VP9 SVC (spatial and temporal layers) and Firefox can render it (even if Firefox can not generate VP9 SVC).
Firefox indeed can render SVC, however, it becomes not smooth if the spatial layer is > 0. I had the same issue with Janus, so making conclusion that SVC on Firefox is useless. Please, let me know if you have any thoughts about that.
No, I do not have any thoughts about it. Something to tell to Firefox developers.