Use this function to create a webRequest.StreamFilter object for a request. Then use the stream filter to monitor and modify the response. You'd typically call this function from a webRequest event listener.

Firefox uses an optimized byte cache for script requests. This optimized byte cache overrides the normal request caching. Data from this cache is not available in a form useful to extensions. If your extension needs to filter scripts, create your filter in webRequest.onBeforeRequest. Doing this ensures that the filter is created prior to the attempt to load from cache, thereby avoiding the optimized cache.

To use this API you must have the "webRequestBlocking" API permission, as well as the normal permissions needed for the event listener (the "webRequest" permission and the host permission for the host).


var filter = browser.webRequest.filterResponseData(
  requestId       // string


string. ID of the request to filter. You can get this from the details object that is passed into any webRequest event listeners.

Return value

A webRequest.StreamFilter object that you can use to monitor and modify the response.


This example, taken from the http-response example extension, creates a filter in webRequest.onBeforeRequest and uses it, to modify the first chunk of the response:

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);
  let decoder = new TextDecoder("utf-8");
  let encoder = new TextEncoder();

  filter.ondata = event => {
    let str = decoder.decode(event.data, {stream: true});
    // Just change any instance of Example in the HTTP response
    // to WebExtension Example.
    str = str.replace(/Example/g, 'WebExtension Example');

  return {};

  {urls: ["https://example.com/*"], types: ["main_frame"]},

Example extensions

Note that this example only works for small requests that aren't chunked or streamed. More advanced examples are documented with webRequest.StreamFilter.ondata.

Browser compatibility

BCD tables only load in the browser