In order to make manual testing easier, it would be very helpful to have access to a browser plugin that allows users to hit their local network or localhost. In theory, this could be implemented as allowing your computer to operate as a proxy which would enable the requests to be routed to your local network. Also, it would be great to enable the ability to upload files from your local folders (for testing things like file uploads).
For example, QAs testing new features could easily hit the internal network to hit pre-production environments and do cross-browser compatibility testing. UI designers could quickly test their new CSS or designs, and developers can easily test their new features in other browsers and operating systems by routing requests to their localhost. All of this eliminates the need to spin up local VMs, and enables a much wider array of compatibility testing options.