This is the site with the credentials - It's not an intermediary/proxy. The point is I want to have a different site (eg. myawesomehnsite.com) that will use th

This is the site with the credentials - It's not an intermediary/proxy. The point is I want to have a different site (eg. myawesomehnsite.com) that will use the firebase api for a reader but allow the user to upvote or comment on my site and proxy that action to news.ycombinator.com. Unless they expose some API to do it, which AFAIK they don't, you don't have a way to do this entirely in the browser. You'll hit cross-site XHR issues or the requests will not be authorized (look at the link for an upvote - it has an auth qs var - this is custom per user per link).