list-resources.js

// -sp-context:browser
// Open this file in scratchpad and run it. Output goes to the Browser Console (Cmd-Shift-J).

Components.utils.import("resource://gre/modules/devtools/dbg-server.jsm");
Components.utils.import("resource://gre/modules/devtools/dbg-client.jsm");
let { devtools } = Cu.import("resource://gre/modules/devtools/Loader.jsm", {});
let { StyleSheetsFront } = devtools.require("devtools/server/actors/stylesheets");

let client;

function startDebugger()
{
  // Start the server.
  if (!DebuggerServer.initialized) {
    DebuggerServer.init();
    DebuggerServer.addBrowserActors();
  }
  // Listen to an nsIPipe
  let transport = DebuggerServer.connectPipe();
  // Start the client.
  client = new DebuggerClient(transport);
  client.connect(onConnected);
}

function shutdownDebugger()
{
  client.close();
}

/**
 * Start debugging the current tab.
 */
function onConnected()
{
  // Get the list of tabs to find the one to attach to.
  client.listTabs(function(response) {
    // Find the active tab.
    let tab = response.tabs[response.selected];
    // Attach to the tab.
    client.attachTab(tab.actor, function(response, tabClient) {
      if (!tabClient) return;
      console.log("HTML:", tab.url);
      // Attach to the thread (context).
      client.attachThread(response.threadActor, function(response, threadClient) {
        if (!threadClient) return;
        // Fetch the JS sources.
        threadClient.getSources(function(response) {
          console.log("JS:", response.sources.map(s => s.url));

          // Fetch the stylesheets.
          let ssf = StyleSheetsFront(client, tab);
          ssf.getStyleSheets().then((styleSheets) => {
            console.log("CSS:", styleSheets.map(s => s.href));
          });
        });
        // Resume the thread.
        threadClient.resume();
        // Debugger is now ready and debuggee is running.
      });
    });
  });
}

startDebugger();
// shutdownDebugger();

It's nice example of how a (RDP) connection to the server can be established. Is there also a simple way how to reuse an existing connection (if any) created by native tools and confirmed by the user, from within an Firefox extension?

Honza

Connections (client endpoints to be precise) are not supposed to be shared among tools. Each tool is expected to create its own connection to the (shared) server. But assuming you just want to augment the native tools, I would say that you need to get a reference to the toolbox first, and get toolbox.target.client from there.