I was investigating Bug 1808868 - backpressure support for fetch API and was looking at the demo provided by the reporter: https://fetch-flow-control-demo.glitch.me/
The demo issues a fetch but only reads from the response when you click the button. It expects to not download the full resource until you actually read from the response.

In Chrome, clicking the Fetch button in the no-store section, immediately displays the 200 status of the fetch request. Clicking read500K updates the Size of the downloaded resource.

In Firefox, we see the pending request, but we have no indication if it succeeded, if it downloaded any bytes or anything.

We should probably update the status in OnStartRequest, then use nsIProgressEventSink.onProgress to update the downloaded/uploaded size of the request, not only when onStopRequest is called.