::selection

Это псевдо-класс не работает в браузере Mozilla Firefox.

css/style.css

p::selection {
color: #FFF;
background-color: #852E76;
}

p::selection {
 color: #FFF;
 background-color: #852E76;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
</head>
<body>
<p>To be or not to be?</p>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
</head>
<body>
<p>To be or not to be?</p>
</body>
</html>