You can observe `Cookie` header with a webRequest.onHeadersReceived listener. Documentation about what can be observed or not is all available online, there is

You can observe `Cookie` header with a webRequest.onHeadersReceived listener. Documentation about what can be observed or not is all available online, there is no need to make any assumption: Firefox: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest/onHeadersReceived https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web... Chromium: https://developer.chrome.com/docs/extensions/reference/webRequest/#life-cycle-of-requests https://developer.chrome.com/docs/extensions/reference/webRe... That should answer all questions regarding what can be observed.