[christon26]

Since you're using postbit and not postbit legacy, and your alt classes have borders styled into them that will make those borders show, try just adding the signature inside a fieldset without adding it into a new table cell as well:

Replace:

        $template_hook[postbit_signature_start]
        $ad_location[ad_showthread_firstpost_sig]

        <if condition="$post['signature']">
        <!-- sig -->
            <div>
                __________________<br />
                $post[signature]
            </div>
        <!-- / sig -->
        </if>

        $template_hook[postbit_signature_end]

With

        $ad_location[ad_showthread_firstpost_sig]

        <if condition="$post['signature']"><br />
        <!-- sig -->

            <fieldset class="signature">
<legend>Signature</legend>
                
                $post[signature]
            </fieldset>
        <!-- / sig -->
        </if>

        $template_hook[postbit_signature_end]

*crosses fingers*