I found the solution. It's to change from "onpopupshowing" to "onpopupshown"
<menupopup id="contentAreaContextMenu" onpopupshown="XULSchoolChrome.BrowserOverlay.test(event)">
<menuitem id="thumbnail-show-etes"
label="My Label"
class="menuitem-iconic"
image="chrome://xulschoolhello/skin/favicon.png"
oncommand="XULSchoolChrome.BrowserOverlay.injectScript('tap_browser.js');"/>
</menupopup>