Is the published stream with VP8 codec? If that's VP8, can you please try to set Mozilla subscription option with VP8 codec? Firefox has defect on receiving stream. If the stream codec is not the first choice codec of Firefox, then refuse to decode.
I am also experiencing this issue.
If I initiate the call from Chrome to Firefox it seems to be working. But if Firefox is the first in call it show black streams.
Will there be some update in a feature release that will fix this issue?