Firefox about:config devtools.debugger

Web Browser Sniffing

function get_browser_name($user_agent)
{
    if (strpos($user_agent, 'opera') || strpos($user_agent, 'opr/')) return 'opera';
    elseif (strpos($user_agent, 'edge')) return 'edge';
    elseif (strpos($user_agent, 'chrome')) return 'chrome';
    elseif (strpos($user_agent, 'safari')) return 'safari';
    elseif (strpos($user_agent, 'firefox')) return 'firefox';
    elseif (strpos($user_agent, 'msie') || strpos($user_agent, 'trident/7')) return 'internet explorer';
    elseif (strpos($user_agent, 'ipad')) return 'ipad';
    
    return 'Other';
}

$userAgent = strtolower($_SERVER['HTTP_USER_AGENT']);
$browser = get_browser_name($userAgent);

(javascript)
var userAgent = '';
var browser = '';
console.log('userAgent '+userAgent);
console.log('browser '+browser);