this stopped working as of firefox 92, most recently tested with 94.0.1
<ul>
	<li>open this in two tabs and click the link</li>
	<li>the tab which sent the message shows the correct value; the receiving tab is <em>usually</em> one message behind</li>
	<li>tick the checkbox <b>on the receiving end</b> to avoid the bug</li>
</ul>

<h1><a href="javascript:send_msg()">send message</a></h1>
<h2>latest msg goes here</h2>

<input type="checkbox" id="avoid_bug" />
<label for="avoid_bug">avoid the bug for incoming messages</label>

<script>
	var chan = new BroadcastChannel("ch"),
		h2 = document.querySelector('h2'),
		avoid_bug = document.querySelector('#avoid_bug');

	function send_msg() {
		localStorage.setItem('msg', Date.now() % 100000);
		chan.postMessage(1);
		show_value();
	}
	
	function show_value() {
		h2.textContent = localStorage.getItem('msg');
	}
	
	chan.onmessage = function (msg) {
		if (avoid_bug.checked) {
			setTimeout(show_value, 1);
		}
		else show_value();
	};
</script>