This event is fired just before sending headers. If your extension or some other extension modified headers in onBeforeSendHeaders, you'll see the modified version here.

This event is informational only.


  listener,             // function
  filter,               //  object
  extraInfoSpec         //  optional array of strings

Events have three functions:

addListener(listener, filter, extraInfoSpec)

Adds a listener to this event.


Stop listening to this event. The listener argument is the listener to remove.


Check whether listener is registered for this event. Returns true if it is listening, false otherwise.

addListener syntax



The function called when this event occurs. The function is passed this argument:


object. Details about the request. See the details section for more information.


webRequest.RequestFilter. A filter that restricts the events that is sent to this listener.

extraInfoSpec Optional

array of string. Extra options for the event. You can only pass one value here:

  • "requestHeaders": include the request headers in the details object passed to the listener

Additional objects



string. If the request is from a tab open in a contextual identity, the cookie store ID of the contextual identity. See Work with contextual identities for more information.


string. URL of the document in which the resource will be loaded. For example, if the web page at "https://example.com" contains an image or an iframe, then the documentUrl for the image or iframe will be "https://example.com". For a top-level document, documentUrl is undefined.


integer. Zero if the request happens in the main frame; a positive value is the ID of a subframe in which the request happens. If the document of a (sub-)frame is loaded (type is main_frame or sub_frame), frameId indicates the ID of this frame, not the ID of the outer frame. Frame IDs are unique within a tab


boolean. Whether the request is from a private browsing window.


string. Standard HTTP method: for example, "GET" or "POST".


string. URL of the resource which triggered the request. For example, if "https://example.com" contains a link, and the user clicks the link, then the originUrl for the resulting request is "https://example.com".

The originUrl is often but not always the same as the documentUrl. For example, if a page contains an iframe, and the iframe contains a link that loads a new document into the iframe, then the documentUrl for the resulting request will be the iframe's parent document, but the originUrl will be the URL of the document in the iframe that contained the link.


integer. ID of the frame that contains the frame which sent the request. Set to -1 if no parent frame exists.


object. This property is present only if the request is being proxied. It contains the following properties:


string. The hostname of the proxy server.


integer. The port number of the proxy server.


string. The type of proxy server. One of:

  • "http": HTTP proxy (or SSL CONNECT for HTTPS)
  • "https": HTTP proxying over TLS connection to proxy
  • "socks": SOCKS v5 proxy
  • "socks4": SOCKS v4 proxy
  • "direct": no proxy
  • "unknown": unknown proxy

string. Username for the proxy service.


boolean. True if the proxy will perform domain name resolution based on the hostname supplied, meaning that the client should not do its own DNS lookup.


integer. Failover timeout in seconds. If the proxy connection fails, the proxy will not be used again for this period.


string. The ID of the request. Request IDs are unique within a browser session, so you can use them to relate different events associated with the same request.

requestHeaders Optional

webRequest.HttpHeaders. The HTTP request headers that have been sent out with this request.


integer. ID of the tab in which the request takes place. Set to -1 if the request isn't related to a tab.


boolean. Indicates whether the request and its content window hierarchy are third party.


number. The time when this event fired, in milliseconds since the epoch.


webRequest.ResourceType. The type of resource being requested: for example, "image", "script", "stylesheet".


string. Target of the request.


object. The type of tracking associated with the request, if the request is classified by Firefox Tracking Protection. This is an object with these properties:


array of strings. Classification flags for the request's first party.


array of strings. Classification flags for the request or its window hierarchy's third parties.

The classification flags include:

  • fingerprinting and fingerprinting_content: indicates the request is involved in fingerprinting ("an origin found to fingerprint").
    • fingerprinting indicates the domain is in the fingerprinting and tracking category. Examples of this type of domain include advertisers who want to associate a profile with the visiting user.
    • fingerprinting_content indicates the domain is in the fingerprinting category but not the tracking category. Examples of this type of domain include payment providers who use fingerprinting techniques to identify the visiting user for anti-fraud purposes.
  • cryptomining and cryptomining_content: similar to the fingerprinting category but for cryptomining resources.
  • tracking, tracking_ad, tracking_analytics, tracking_social, and tracking_content: indicates the request is involved in tracking. tracking is any generic tracking request, the ad, analytics, social, and content suffixes identify the type of tracker.
  • any_basic_tracking: a meta flag that combines tracking and fingerprinting flags, excluding tracking_content and fingerprinting_content.
  • any_strict_tracking: a meta flag that combines all tracking and fingerprinting flags.
  • any_social_tracking: a meta flag that combines all social tracking flags.

Browser compatibility

BCD tables only load in the browser


This code logs all cookies that will be sent in making requests to the target match pattern:

// The target match pattern
let targetPage = "*://*.google.ca/*";

// Log cookies sent with this request
function logCookies(e) {
  for (const header of e.requestHeaders) {
    if (header.name === "Cookie") {

// Listen for onSendHeaders, and pass
// "requestHeaders" so we get the headers
  { urls: [targetPage] },

Note: This API is based on Chromium's chrome.webRequest API. This documentation is derived from web_request.json in the Chromium code.