Fired when a request could not be processed due to an error: for example, a lack of Internet connectivity.

The error is passed to the listener as the error property of the details object.

Note that this event is not fired for HTTP errors (4XX or 5XX responses): these will go through the normal stages of a request, calling any event listeners, and setting details.statusCode to report the error.

This event is informational only.



  listener,             // function
  filter                //  object

Events have three functions:

addListener(listener, filter)

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.

Additional objects



string. If the request is from a tab open in a contextual identity, the cookie store ID of the contextual identity.


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.


string. The error description. This string is an internal error string, may vary from one browser to another, and is not guaranteed to stay the same between releases.


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. Indicates if this response was fetched from disk cache.


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


string. The IP address of the server the request was sent to. It may be a literal IPv6 address.


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.


integer. The 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 with the request has been 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. fingerprinting_content indicates the request is loaded from an origin that has been found to fingerprint but is not considered to participate in tracking, such as a payment provider.
  • 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 any tracking and fingerprinting flags, excluding tracking_content and fingerprinting_content.
  • any_strict_tracking: a meta flag that combines any tracking and fingerprinting flags, including tracking_content and fingerprinting_content.
  • any_social_tracking: a meta flag that combines any social tracking flags.

Note If Firefox Tracking Protection blocks the request an empty object is returned and error returns one of these codes:

  • NS_ERROR_MALWARE_URI indicating a malware URI.
  • NS_ERROR_PHISHING_URI indicating a phishing URI.
  • NS_ERROR_TRACKING_URI indicating a tracking URI.
  • NS_ERROR_UNWANTED_URI indicating an unwanted URI.
  • NS_ERROR_BLOCKED_URI indicating a blocked URI.
  • NS_ERROR_HARMFUL_URI indicating a harmful URI.
  • NS_ERROR_FINGERPRINTING indicating a fingerprinting URI.
  • NS_ERROR_CRYPTOMINING_URI indicating a cryptomining URI.
  • NS_ERROR_SOCIALTRACKING_URI indicating a social tracking URI.

Browser compatibility

BCD tables only load in the browser



let target = "<all_urls>";

e.g., with no network:
function logError(responseDetails) {

browser.webRequest.onErrorOccurred.addListener(logError, { urls: [target] });

Example extensions

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