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:
User-agent: Twitterbot
Allow: /
to our 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.