You can use this event to modify HTTP response headers. webRequest.onHeadersReceivedįired when the HTTP response headers associated with a request have been received. If your add-on or some other add-on modified headers in onBeforeSendHeaders, you'll see the modified version here. webRequest.onSendHeadersįired just before sending headers. This is a good place to listen if you want to modify HTTP request headers. webRequest.onBeforeSendHeadersįired before sending any HTTP data, but after HTTP headers are available. This is a good place to listen if you want to cancel or redirect the request. Modifying requestsįired when a request is about to be made, and before headers are available. For example, if a page at loads an image from, then an extension must have both host permissions if it is to intercept the image request. To intercept resources loaded by a page (such as images, scripts, or stylesheets), the extension must have the host permission for the resource as well as for the main page requesting the resource. To use the 'blocking' feature, the extension must also have the 'webRequestBlocking' API permission. To use the webRequest API for a given host, an extension must have the 'webRequest' API permission and the host permission for that host. It stays the same throughout a request, even across redirections and authentication exchanges. It is unique within a browser session and the add-on's context. This includes a request ID, which is provided to enable an add-on to correlate events associated with a single request. The listener function is passed a details object containing information about the request. You can use this to pass additional event-specific instructions. a filter object, so you can only be notified for requests made to particular URLs or for particular types of resource.Also, note that sometimes the sequence of events may differ from this: for example, in Firefox, on an HSTS upgrade, the onBeforeRedirect event will be triggered immediately after onBeforeRequest.Īll events- except onErrorOccurred-can take three arguments to addListener(): OnErrorOccurred can be fired at any time during the request. The typical sequence of events is like this: The event listener receives detailed information about the request and can modify or cancel the request.Įach event is fired at a particular stage of the request. MAX_HANDLER_BEHAVIOR_CHANGED_CALLS_PER_10_MINUTESĪdd event listeners for the various stages of making an HTTP request, which includes websocket requests on ws:// and wss://.Differences between API implementations.

Firefox browser extension web monitor