Late reply, but the UI browsing view/hierarchy on the Roku SoundBridge is fixed. Firefly can’t control it. This is the same for both RSP and DAAP. When you move through the UI, it just sends appropriate query requests to the server.
When Ron implements a UPnP AV server in Firefly eventually, he will be able to have some of this browse “control” via Server Containers…. but you’d be using the slower UPnP AV protocol when that happens. (Roku has a ‘server container’ bypass function, “Browse By Server Containers” — which is the default, if the UPnP AV server doesn’t offer advanced search functionality for the native UI.)