Overwrite the Host HTTP header in Firefox requests to debug your web server or work around DNS restrictions.
This extension is available on Firefox extensions: https://addons.mozilla.org/firefox/addon/vhost-simulator/
Often enough while waiting for DNS propagation or standing up a replacement site, developers edit the hosts file on their system to achieve the same effect. This extension basically does the same thing without the hassle.
When detecting outgoing HTTP requests from your Firefox browser to the VHost IP it will rewrite the Host header to the VHost Domain. Any requests to sites other than the VHost IP are ignored, so browsing other sites won't be impacted.