Instead of showing the envelope icon for the submit button, the unicode text “” is shown instead. The cause is in the Newsletter Plugin’s subscription.php file at line 1478. The unicode “” is passed through the esc_attr() function which resulted in “” being used in the HTML. Removing the esc_attr() from that line got the envelop to display.
A screencast video detailing the bug is posted on YouTube.
1475 if (strpos($options_profile['subscribe'], 'http://') !== false) {
1476 $buffer .= '<input class="tnp-submit-image" type="image" src="' . esc_attr($options_profile['subscribe']) . '">' . "\n";
1477 } else {
1478 // TTT: esc_attr() changes & sign to & when used with Font Awesome values
1479 // like  into "" and cause the icon not to show up
1480 //$buffer .= '<input class="tnp-submit" type="submit" value="' . esc_attr($options_profile['subscribe']) . '">' . "\n";
1481 $buffer .= '<input class="tnp-submit" type="submit" value="' . $options_profile['subscribe'] . '">' . "\n";
1482 }