Hi there, I'm a Frontend developer using ExtJS (3.4.x). After updading / upgrading to Firefox 36.0 I've (and all of my other colleagues working in Frontend development) noticed that having Firebug (2.0.8 AND 3.0.9 alpha) open in a tab when loading our applications makes Firefox unresponsive for 30-180 seconds and sometimes even more. We sometime receive the "Unresponsive script window". Click on "Continue" and after waiting and waiting the page will eventually load and afterwards, everything seems fine. Everything seems fines, until we reload the page... Re-loading the same page will again cause Firefox unresponsiveness for a long time, sometimes showing the "Unresponsive script window". We've uninstalled Firebug and tried the same with "Firefox Developer Tools" only. The same behavior :( It's very slow. Sorry, but we cannot do Frontend development this way :( By closing the Firebug / Firefox Developer Tools window everything turns to normal. Quick load of pages. Opening Firebug / Firefox Developer Tools after the page has been loaded works fine. But one can forget to close Firebug / Firefox Developer Tools before pressing CTRL+F5 to reload the page with the latest changes to the Frontend and this causes a lot of frustration because of the long loading times. Firefox 36.0.1 does not make it better. Everything worked well with lower versions of Firefox ! Everything is OK when working with Google Chrome (latest version x64) or Interne Explorer (latest version 32bit on Windows 8.1 latest update) with or without the Developer Tools open. Private mode / Secure mode of Firefox did not solve the issue. Uninstalling Firefox 36.0 / 36.0.1 and re-installing with clean profile did not solve the issue. Here is an example site that loads like a charm when Firebug / Firefox Developer Tools are not enabled before starting to load the page: http://dev.sencha.com/deploy/ext-3.4.0/examples/desktop/desktop.html Please help me / us with this issue. Any advice is welcome. We're desperate ! Development of our Frontend using Google Chrome / Internet Explorer is such a NO GO. Firebug is our preferred tool and we do not want to miss its comfort features. Thank you so much. Kind Regards, Mihai