I tried to play around with Firebase as well and ran in to the same issue. I don’t think you’re going to be able to get it working because Firebase loads JS from Firebase dynamically (I think?) and the CSP the Iframe is running in only allows JS to be loaded from the Twitch CDN.
There might be a way to get it working though. I only messed with it for a bit before giving up because in the end they’re probably going to deny Firebase in the approval process due to un-readability/“obfuscation”.