Open a Firefox devtools window that's debugging the devtools window that's debugging the currently selected tab.

debug-devtools.js

/*
 * Running this scratchpad in chrome mode will open a devtools window that's debugging
 * the devtools window that's debugging the currently selected tab.
 */
let {devtools} = Components.utils.import("resource:///modules/devtools/gDevTools.jsm", {});
 
let tabTarget = devtools.TargetFactory.forTab(gBrowser.selectedTab);
let toolbox = gDevTools.getToolbox(tabTarget);
 
let win = toolbox._host.frame.contentWindow;
let target = devtools.TargetFactory.forWindow(win);
 
let toolbox = gDevTools.showToolbox(target, "styleeditor", "window");

In nightly, can run this command by:

1. Tools > Web Developer > Browser Console
2. Copy script above and press enter