The SMF session last forever unless you logout or shutdown your computer. So when you come back, you'll find that your still logged. However, the mambo session only last for a period of time. If you want, you can remove the "forever" dropdown in the login.

I've changed the loginform to:
<td colspan="4"><form action="', $scripturl, '?action=mambo_login" method="post" style="margin: 3px 1ex 1px 0; text-align:'.$smf_align.';">
            <input type="hidden" class="button" name="message" value="',$message_login,'" />
            <input type="hidden" name="return" value="',$mosConfig_live_site,'/',basename($_SERVER['PHP_SELF']),'?',$_SERVER['QUERY_STRING'],'" />
            <input type="hidden" class="button" name="lang" value="',$mosConfig_lang,'" />
            <input type="hidden" name="returnurl" value="', $params->get('login'),'" />
            <input type="hidden" name="cookielength" value="-1">
            <input type="hidden" name="op2" value="login" />
            Gebruikersnaam:
            <input type="text" class="inputbox" name="user" size="10" />
            Pass:
            <input type="password" class="inputbox" name="passwrd" size="10" />
            <input type="submit" class="button" value="', $txt[34], '" /></form></td>

so everone is logged in forever until they logout out by clicking the logoutbutton!

Why always fixing things 100 % for Mozilla and Firefox, more than 95 % of visitors are using IE