This add-on lets you send remote debugging protocol requests to the debugging server, and displays the response.
To use it, just type the JSON message into the textbox and hit Ctrl+Enter. The response is displayed on the right-hand side.
For example, try:
{}
or:
{
"to": "root",
"type": "listTabs"
}
To learn more about how this add-on works see the tutorial at: https://developer.mozilla.org/en-US/docs/Tools/Adding_a_panel_to_the_toolbox.