Do you mean that it would be exposed by the referrer? I guess so. It’s not clear whether the referrer is preserved anyway when you refresh the external page in a different container. In theory could the external page have a Facebook widget that would be able to see your Facebook account anyway? I’m not really sure, I don’t know enough about how cookies work across domains.

I feel like this is the point. It’s not really clear how to best use containers to protect your privacy; it feels like you have to understand a lot about how the web works and how Firefox works.