I did some research and to call a custom field, you need to have the {$user.customFields.fieldID} parameter, but when I put it into the code (such as below, my fieldid is firstname). It outputs only.
| Castile, not Alex | Castile. Help?
| Castile, not Alex | Castile. Help?
<xen:if is="{$user.user_id}">
<a href="{xen:link members, $user}"
class="username{xen:if '!{$user.visible}', ' invisible'}{xen:if {$user.followed}, ' followed'}">{$user.customFields.firstname} | {$user.username}</a><xen:if is="{$i} < {$onlineUsers.limit}">,</xen:if>
<xen:else />