Cu.import("resource://gre/modules/jsdebugger.jsm");
addDebuggerToGlobal(this);

let browser = gBrowser.getBrowserForTab(gBrowser.selectedTab);
let contentWindow = browser.contentWindow;

let dbg = new Debugger(contentWindow);

let start = Date.now();
dbg.memory.captureDominatorTree(contentWindow)
let end = Date.now() - start;
end