My extension changes request headers in the nsIHttpChannel. Now, I would
like to indicate those changes in the status bar. However, I don't find
an appropriate path to transfer information from the backend (modifying
the headers) to the frontend (status bar).

Ideally, there would be a way to attach data to the nsIHttpChannel and
access that data from the DOM document.

Do you happen to know how? Any links/hints are appreciated.