UPDATE 12/20: This should now just work. Horray!
Firebase Storage PM here.
Sorry you've hit this issue--I'm honestly surprised this hasn't come up before. Looks like to fix this, we'd have to add:
robots.txt. I'm talking with the eng team about this now, but it seems feasible.
Another option might be to serve our content through a CDN (like Fastly, see GCS + Fastly), at which point you can add your own
robots.txt set to whatever you'd like.