Openings in server firewall for Leaflet and Bing Maps


I have developed an app that uses Leaflet and Bing Maps and it will be deployed in a server behind a firewall.
I have to ask the admin to open the urls that will be used by leaflet to download the tiles from Bing Maps, but I am not quite sure how this works. The request is sent by the server or by the browser? Because the browser (so the user pc) has no restraints, while the server is completely offline except for the urls I require.
In other words, should I ask for the openings to
or does the browser connect to them directly?

Thanks in advance!