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If you don't see the test messages in Firefox then install the FirePHP extension and enable it for your site.
If you don't see the test messages in Chrome then install the FirePHP extension and enable it for your site.

HSFirePHP!::LOG

The following code is used to generate the demo:

 FB::group('HSFirePHP!::LOG', array('Color' => '#0000FF'));
 FB::log('Plain Message');
 FB::info('Info Message');
 FB::warn('Warn Message');
 FB::error('Error Message');
 FB::log('Message','Optional Label');
 FB::groupEnd();

You will see 5 messages (log, info, warn, error, log with optional label):

HSFirePHP!::ERROR

The following code is used to generate the demo:

FB::group('HSFirePHP!::ERROR', array('Color' => '#0000FF'));
if (version_compare(phpversion(), '8.0.0.all', '<'))
{
    trigger_error('', E_ERROR);
}
else
{
    $E_WARNING++;
}
if (version_compare(phpversion(), '8.0.0.all', '<'))
{
    $E_NOTICE++;
}
else
{
    date_default_timezone_set('INVALID');
}
trigger_error('E_USER_ERROR', E_USER_ERROR);
trigger_error('E_USER_WARNING', E_USER_WARNING);
trigger_error('E_USER_NOTICE');
if (version_compare(phpversion(), '8.0.0.all', '<'))
{
    mb_ereg_replace('', '', '', 'e');
}
else
{
    $array = [0, 1, 2];
    usort($array, function (int $a, int $b): bool {
        return $a > $a;
    });
}
trigger_error('E_USER_DEPRECATED', E_USER_DEPRECATED);
FB::groupEnd();

You will see 7 errors (E_WARNING, E_NOTICE, E_USER_ERROR, E_USER_WARNING, E_USER_NOTICE, E_DEPRECATED, E_USER_DEPRECATED):

HSFirePHP!::EXCEPTION

Execution will stop after the exception handler is called. You can test this here. Joomla! 4.x will try to catch exceptions and display an own error page.
Warning: In Joomla! 3.x you will no longer be able to render the page this tag is used in.

The following code is used to generate the demo:

if (4 !== Version::MAJOR_VERSION)
{
    FB::group('HSFirePHP!::EXCEPTION', array('Color' => '#0000FF'));
}
throw new Exception('Uncaught Exception');

You will see 1 uncaught exception:

HSFirePHP!::ASSERTION

The following code is used to generate the demo:

FB::group('HSFirePHP!::ASSERTION', array('Color' => '#0000FF'));
assert(true == false);
FB::groupEnd();

You will see 1 assertion (true == false) with result FALSE in Joomla! 3.x: