If/Else not working for header

You actually doing mistake. You have to pass 3 parameters as array not like this.
Use below code

<?php //if is single event, guide, firm news
    if ( is_singular( array('events','guide','firm-news') ) ) : ?>
        <h5 class="event-subpage-header-title"><?php echo $title; ?></h5>
<?php 
    else : 
?>
        <h1><?php echo $title; ?></h1>
<?php 
    endif; 
?>