Firefox has a different implementation of ICE than Chrome, so if your
configuration is incomplete, it's not impossible for one browser to
succeed and the other to fail.
In the present case, It is the contrary : all browsers succeed but firefox on some networks (mainly 4G SFR)
What does Galène say about the relay test in the log?
I did try to reproduce using firefox on my IPAD with a 4G , I first got
"pc ERROR: 2021/05/01 15:02:05 Incoming unhandled RTP ssrc(279383751), OnTrack will not be fired. mid RTP Extensions required for Simulcast »
But the second try did work. Strange.
Cheers,
Fabrice.