var event = new pTab.linkedBrowser._contentWindow.CustomEvent("prefwindow-event");
edit:
Proof of concept, open scratchpad, switch environment to browser, run the following snippet
var url = 'data:text/html;charset=utf-8,';
var content = '<html><script>addEventListener("Hello",function(){alert("Hello")},false)</script></html>'
var tab = gBrowser.loadOneTab(url+encodeURIComponent(content), {inBackground: false});
tab.addEventListener("load", function(){
var evt = new tab.linkedBrowser.contentWindow.CustomEvent("Hello");
tab.linkedBrowser.contentWindow.dispatchEvent(evt);
}, false);