webRequest.StreamFilter.disconnect()

Disconnects the filter from the request. After this, the browser will continue to process the response, but no more filter events will fire, and no more filter function calls will have any effect. Note the difference between this function and close(). With disconnect(), the browser will continue to process any further response data, but it won't be accessible through the filter. With close(), the browser will ignore any response data that hasn't already been passed through to the rendering engine.

You should always call disconnect() or close() once you don't need to interact with the response any further.

You can't call this function until after the onstart event has fired.

filter.disconnect()

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);

  filter.onstart = (event) => {
    console.log("started");
    let encoder = new TextEncoder();
    filter.write(encoder.encode("preface text"));
    filter.disconnect();
  }
}

browser.webRequest.onBeforeRequest.addListener(
  listener,
  {urls: ["https://example.org/"], types: ["main_frame"]},
  ["blocking"]
);