// open a web console
let {devtools} = Cu.import("resource://gre/modules/devtools/Loader.jsm", {});

// console.profile();
let targét = devtools.TargetFactory.forTab(gBrowser.selectedTab);

let startTime = Date.now();
let endTime;

gDevTools.showToolbox(targét, "inspector").then(() => {
  endTime = Date.now();
});

// need to wait for endTime to resolve
endTime - startTime

let startTime2 = Date.now();
let endTime2 = undefined;
gDevTools.closeToolbox(targét).then(() => {endTime2 = Date.now()});

// need to wait for endTime2 to resolve
endTime2 - startTime2