When you navigate to a web site which is slow to respond the address bar keeps showing the previous address until the web page loads or a network error occurs. It may take minutes. It happens for instance when clicking a a web search result or when the site itself redirects to another web page. This Firefox behavior is there for years and I even remember that Internet Explorer had the same behavior during its era. Is there any particular reason to do that? Because I only see downsides: for instance, you can't copy the URL to check in another program or device. Somebody might argue that it's because you can press Esc during load and stay on the previous page. I think it would be more convenient to revert the address bar value when this happens.