firefoxPref can't change javascript.enabled

Hello,

I have tried to measure a performance of a website without JS. I don’t want just block js requests, but simulate that JS is disabled in the browser.
So I gave a try to https://sites.google.com/a/webpagetest.org/docs/using-webpagetest/scripting#TOC-firefoxPref - the setting itself seems to be working fine (e.g. firefoxPref general.useragent.override “TEST” is ok).
But I do not see any difference between javascript.enabled (http://kb.mozillazine.org/Javascript.enabled) true or false:
http://www.webpagetest.org/result/151123_GE_d8f9a2b8c72eb2b4c8dcb4c3c5223e3c/1/details/

firefoxPref javascript.enabled true navigate https://www.mozilla.org/en-US/firefox/new/
http://www.webpagetest.org/result/151123_Y4_eefca052be345cb6f3f3f88f5ab77e0e/1/details/

firefoxPref javascript.enabled false navigate https://www.mozilla.org/en-US/firefox/new/

Two questions:

  1. Is there anything specific about javascript.enabled? Is this an expected behavior?
  2. Any recommendation how to simulate javascript disabled?

Ok, there is an easier - trivial way - to disable JS. Trying to solve my issue I found https://github.com/WPO-Foundation/webpagetest/blob/master/agent/wptdriver/web_browser.cc and this piece of code if (_test._noscript) { user_prefs += "user_pref(\"javascript.enabled\", false);\r\n"; }
led me to Advanced Settings > Advanced > Disable Javascript checkbox on www.webpagetest.org