nsIProtocolProxyService
if the protocol handler for the given uri supports nsiproxiedprotocolhandler, then the nsiproxyinfo instance returned from resolve may be passed to the nsiproxiedprotocolhandler.newproxiedchannel() method to create a nsichannel to the given uri that uses the specified proxy.
... however, if the nsiproxyinfo type is "http", then it means that the given uri should be loaded using the http protocol handler, which also supports nsiproxiedprotocolhandler.
...see nsiproxiedprotocolhandler.newproxiedchannel().
... see also nsiproxyinfo nsiprotocolproxyfilter nsiproxiedprotocolhandler nsicancelable ...

nsIProtocolHandler
if the protocol supports transparent proxying, the handler should implement the nsiproxiedprotocolhandler interface.
... if the protocol only supports http proxying, it doesn't need to support nsiproxiedprotocolhandler, but should instead set the allows_proxy_http flag.
... see also nsiproxiedprotocolhandler adding a new protocol to mozilla writing a firefox protocol handler custom about: urls - similar to adding new protocol but adding onto the existing about: protocol ...

Proxies in Necko
this means that callers can just create an nsichannel, not needing to worry about whether the channel will use a proxy or not the basic interfaces for proxies are: nsiproxyinfo, nsiprotocolproxyservice, and nsiproxiedprotocolhandler nsiproxyinfo is a simple helper which stores information about the type of the proxy, its host and its port.
...depending on the type of the proxy info, it either asks the http protocol handler or the protocol handler for the requested uri for a new channel with the nsiproxyinfo - if the channel supports nsiproxiedprotocolhandler.

nsISocketTransportService
for more details on communicating information about proxies like socks (which are transparent to upper protocols), see nsiproxiedprotocolhandler , nsiprotocolproxyservice or proxies in necko.