As of June 2020, there is no official method/way of achieving http request requester filtering/identification.
Currently the only possibility is what is done on the question’s code which is separating requests from web-pages/tabs and the other Firefox’s components (feed updates, extensions requests, XHR requests from XPCOM components, etc).
As mentioned on the comments this is an internal limitation of Firefox. The current Firefox’s core code does not implement a requester tracking and thus does not know who initiated the request and why. It may be useful to know that Chrome dev tools got recently this feature.