Actually I looked at your page source and I think your confusing IE.. Mozzilla doesn't get fooled easily..
See this is yours..
<table width="98%" border="0" align="center" cellpadding="0" cellspacing="0" background="http://www.pilotfamilies.com/forums/stylesheets/images/koniro/bkg2.png">
<tr>
<td width="57%" height="63" align="left" valign="middle"><div align="right"><img src="/images/PF_logo.gif" width="260" height="60" align="left">
<a href='http://www.jetcareers.com/phpads/adclick.php?n=a9c97f8c' target='_blank'><img src='http://www.jetcareers.com/phpads/adview.php?what=zone:12&n=a9c97f8c' alt='' border='0' align="middle"></a></div></td>
</tr>
</table>
<!-- START OF header.tmpl TEMPLATE -->
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html dir="ltr">
<head>
<meta name="robots" content="index, follow" />
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<link rel="stylesheet" href="http://www.pilotfamilies.com/forums/stylesheets/koniro.css" type="text/css" />
<link rel="shortcut icon" href="http://www.pilotfamilies.com/forums/images/favicon.ico" />
<title>pilotfamilies.com: Viewing list of forums</title>
</head>
<body>
<!-- END OF header.tmpl TEMPLATE -->
this is most other sites
<!-- START OF header.tmpl TEMPLATE -->
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html dir="ltr">
<head>
<meta http-equiv="PICS-Label" content='(PICS-1.1 "http://www.icra.org/ratingsv02.html" l gen true for "https://ubbdev.com" r (cz 1 lz 1 nz 1 oz 1 vz 1) "http://www.rsac.org/ratingsv01.html" l gen true for "https://ubbdev.com" r (n 0 s 0 v 0 l 0))' />
<meta name="description" content="Where UBB.Threads developers come for answers." />
<meta name="keywords" content="php, mysql, bulletin boards, forums, BBS, internet, online, discussion groups, newsgroups, ubb.threads, phpbb, vb, vbulletin, UBB, wwwthreads, ultimate bulletin board, customize, Perl, CGI, UNIX, NT, affordable, hacking, Infopop, community, UltimateBB, scripts, phpnuke, postnuke, threads, topics" /><meta name="robots" content="index, follow" />
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<link rel="stylesheet" href="https://ubbdev.com/forum/stylesheets/orangedarkness.css" type="text/css" />
<link rel="shortcut icon" href="https://ubbdev.com/forum/images/favicon.ico" />
<title>Threadsdev.com - UBB.Threads Modifications: Listing categories</title>
</head>
<body>
<!-- END OF header.tmpl TEMPLATE -->
you may be confusing IE upon first look but it doesn't stay fooled twice so it passes it... can you try moving the
<table width="98%" border="0" align="center" cellpadding="0" cellspacing="0" background="http://www.pilotfamilies.com/forums/stylesheets/images/koniro/bkg2.png">
<tr>
<td width="57%" height="63" align="left" valign="middle"><div align="right"><img src="/images/PF_logo.gif" width="260" height="60" align="left">
<a href='http://www.jetcareers.com/phpads/adclick.php?n=a9c97f8c' target='_blank'><img src='http://www.jetcareers.com/phpads/adview.php?what=zone:12&n=a9c97f8c' alt='' border='0' align="middle"></a></div></td>
</tr>
</table>
to inside the header? this appears to be your actual header. If ya still want it floating try putting it before any table calls in the header.php in your includes directory. See if that fixes this issue with IE...
