event-handler-xbl-bubbling.js

// Doesn't bubble to parentNode, but does fire on gBrowser (the bindingParent for gBrowser.selectedBrowser)
gBrowser.addEventListener("bar", () => { console.log("No explicit bubbling, but bubbled to gBrowser") });
gBrowser.selectedBrowser.parentNode.addEventListener("bar", () => { console.log("No explicit bubbling, did not bubble to parentNode") });
gBrowser.selectedBrowser.dispatchEvent(new CustomEvent("bar"));

// If you tell the CustomEvent to bubble, then it does bubble to the parentNode (and gBrowser):
gBrowser.addEventListener("bar", () => { console.log("Explicit bubbling, did bubble to gBrowser") });
gBrowser.selectedBrowser.parentNode.addEventListener("bar", () => { console.log("Explicit bubbling, did bubble to parentNode") });
gBrowser.selectedBrowser.dispatchEvent(new CustomEvent("bar", { bubbles: true }));