Text Editor Missing

[Mipher]

Try this headerinclude:

<link rel="alternate" type="application/rss+xml" title="{$lang->latest_threads} (RSS 2.0)" href="{$mybb->settings['bburl']}/syndication.php" />
<link rel="alternate" type="application/atom+xml" title="{$lang->latest_threads} (Atom 1.0)" href="{$mybb->settings['bburl']}/syndication.php?type=atom1.0" />
<meta http-equiv="Content-Type" content="text/html; charset={$charset}" />
<meta http-equiv="Content-Script-Type" content="text/javascript" />

{$stylesheets}
<link rel="stylesheet" type="text/css" href="cache/themes/global/thanks/thx.css" />
<link rel="stylesheet" href="{$mybb->settings['bburl']}/images/df/font/font-awesome.min.css">
<link rel="stylesheet" href="{$mybb->settings['bburl']}/images/df/font/font-awesome-ie7.min.css">

<script type="text/javascript" src="{$mybb->asset_url}/jscripts/jquery.js?ver=1800"></script>
<script type="text/javascript" src="{$mybb->asset_url}/jscripts/jquery.plugins.min.js?ver=1800"></script>
<script type="text/javascript" src="{$mybb->asset_url}/jscripts/general.js?ver=1800"></script>
<script type="text/javascript">
<!--
 lang.unknown_error = "{$lang->unknown_error}";

 lang.select2_match = "{$lang->select2_match}";
 lang.select2_matches = "{$lang->select2_matches}";
 lang.select2_nomatches = "{$lang->select2_nomatches}";
 lang.select2_inputtooshort_single = "{$lang->select2_inputtooshort_single}";
 lang.select2_inputtooshort_plural = "{$lang->select2_inputtooshort_plural}";
 lang.select2_inputtoolong_single = "{$lang->select2_inputtoolong_single}";
 lang.select2_inputtoolong_plural = "{$lang->select2_inputtoolong_plural}";
 lang.select2_selectiontoobig_single = "{$lang->select2_selectiontoobig_single}";
 lang.select2_selectiontoobig_plural = "{$lang->select2_selectiontoobig_plural}";
 lang.select2_loadmore = "{$lang->select2_loadmore}";
 lang.select2_searching = "{$lang->select2_searching}";

 var cookieDomain = "{$mybb->settings['cookiedomain']}";
 var cookiePath = "{$mybb->settings['cookiepath']}";
 var cookiePrefix = "{$mybb->settings['cookieprefix']}";
 var deleteevent_confirm = "{$lang->deleteevent_confirm}";
 var removeattach_confirm = "{$lang->removeattach_confirm}";
 var loading_text = '{$lang->ajax_loading}';
 var saving_changes = '{$lang->saving_changes}';
 var use_xmlhttprequest = "{$mybb->settings['use_xmlhttprequest']}";
 var my_post_key = "{$mybb->post_code}";
 var rootpath = "{$mybb->settings['bburl']}";
 var imagepath = "{$theme['imgdir']}";
   var yes_confirm = "{$lang->yes}";
 var no_confirm = "{$lang->no}";
 var MyBBEditor = null;
 var spinner_image = "{$theme['imgdir']}/spinner.gif";
 var spinner = "<img src='" + spinner_image +"' alt='' />";
// -->
</script>
<script type="text/javascript" src="images/df/js/df.js"></script>
<script type="text/javascript">
$(function() {
    $('.dropDownHook span').on('click', function() {
        $(this).parents('.dropDownHook').children('.dropDownContent').stop(true, true).slideToggle('fast', function() {

            if ($('.dropDownContent').is(':visible')) {
                //  This alters the content of the "hook" when we open the drop down
                $('.dropDownHook span').html('Welcome {$mybb->user['username']} ▲');
            }
            else {
                //  This alters the content of the "hook" when we close the dropdown
                $('.dropDownHook span').html('Welcome {$mybb->user['username']} ▼');
            }

        });
    });
    
    $('html').on('click', function() {
        $('.dropDownContent:visible').slideToggle('fast');
    });

    $('.dropDownHook > *').on('click', function(event) {
        event.stopPropagation();
    });
});
</script>
<script type="text/javascript" src="jscripts/thx.js"></script>
{$myprofile_headerinclude}
<script type="text/javascript">
$(function() {
    $(".stab_content").hide(); 
    $("ul.stabs li:first").addClass("active").show(); 
    $(".stab_content:first").show();
    $("ul.stabs li").click(function() {
        $("ul.stabs li").removeClass("active"); 
        $(this).addClass("active"); 
        $(".stab_content").hide();
        var activeTab = $(this).find("a").attr("href");
        $(activeTab).fadeIn(); 
        return false;
    });
});
</script>

Are you using a template for the 1.6.x series? I suggest you to use one compatible with the 1.8.x series.