Chrome:
One way to do this in Chrome is to copy the request as cURL in the "Network" panel of the developer tools and play it with your changes in the terminal, if you have the curl command. See Capture:

If you are looking for an all-in-browser workflow, you can install an extension called "Devtools terminal" that will embed your local terminal in the browser.
Firefox:
Firefox has a built-in. You can make your changes and send XHR directly from the Network panel. The bottom capture is taken from Firefox 36:

Michael P. Bazos Feb 27 '15 at 10:21 2015-02-27 22:21
source share