1707418 - response headers modified by extensions do not show up in network monitor

Status: RESOLVED DUPLICATE of bug 1554345

(DevTools :: Netmonitor, defect, P3)

Description

[u669213]

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:89.0) Gecko/20100101 Firefox/89.0

Steps to reproduce:

Install Header Editor extension.
(https://addons.mozilla.org/en-US/firefox/addon/header-editor/?utm_source=addons.mozilla.org&utm_medium=referral&utm_content=search)

Example rule:
name: googletagmanager last-modified
rule type: Modify response header
match type: url prefix
match rules: https://www.googletagmanager.com/gtag/js?id=
header name: last-modified
header value: Fri, 23 Apr 2021 13:21:08 GMT

Go to some site that has such url in network requests during page load like:
https://www.klix.ba/vijesti/bih/dvojica-policajaca-pretukla-mladica-u-mostaru-on-kaze-da-je-samo-krsio-epidemioloske-mjere/210412003

or directly visit url:
https://www.googletagmanager.com/gtag/js?id=UA-63707-9

Actual results:

You will not see last-modified response header in network monitor of developer tools. But in about:cache?storage=disk or by using nirsoft mzcacheview you will. Meaning that extension really did it's job, but it's impossible to see it by developer tools.

Expected results:

It should have displayed last-modified field added by extension to response headers.
Modifying request headers is not problematic tough.

Comment 1

[Hubert Boma Manilla (:bomsy)]

Thanks for reporting! I can reproduce the issue.