I’m using Ruby’s library for getting http pages (net/http), for example:
Net::HTTP.get URI.parse(uri)
Is this visible for user somehow? I mean, can the user use firebug (for example) to obtain uri or is this is only handled and visible by the server?
No, Net::HTTP requests are on the server running Ruby. The user cannot monitor those requests unless they had access to the server or the server’s network.