Page request fails to load debug toolbar.

I've tried all suggestions here and in SO.

.env: CI_ENVIRONMENT = development
app/Config/App.php: public $baseURL = 'http://localhost/MZCI4/public/';

If I place echo "First controller test";  at top of controller index() function, it appears on blank page immediately after I enter the URL into the browser address bar.

Then CI4 waits about 4 seconds and then shows the actual requested page, correctly. But no debug icon shows at lower right corner.

Firefox "Inspect Element", network view, issues a warning at bottom: "Loading failed for the <script> with source “http://localhost/MZCI4/public/index.php?debugbar".

The network timing view shows 3 GETs. " 3 requests 30.94 KB / 31.24 KB transferred Finish: 6.57 s DOMContentLoaded: 6.55 s
load: 6.59 s". It pauses on the middle Get, to load the debug toolbar, for about 4 seconds. It then completes the page load and issues the warning.

The inspector view shows the html for making the requests.

Any help is appreciated.