I'm having the same problem, it works fine in IE, but doesn't work in FireFox. Here is an example:
<atc:AccordionPane ID="AccordionPane2" runat="server" ContentCssClass="AccordionBody"
HeaderCssClass="AccordionHeader">
<Header>
<asp:LinkButton ID="LinkButton15" runat="server" onmouseover="pop(this);">
<div onmouseout="this.style.backgroundColor = '';" onmouseover="this.style.backgroundColor = '#FDCFAA';"
style="width: 98.5%; cursor: pointer; padding-top: 10px; padding-bottom: 10px;" onclick="DemosClick();">
<div style="width: 98%; height: 20px;">
<asp:LinkButton ID="LinkButton16" runat="server">
<twc:Rollover ID="Rollover4" runat="server" BorderWidth="0" ImageUrl="images/title_demos.gif"
RollOverImageUrl="images/title_demos.gif" Style="float: left;"
ToolTip="View Details.."></twc:Rollover>
</asp:LinkButton>
<asp:LinkButton ID="LinkButton17" runat="server">
<asp:Image ID="DemosExpand" runat="server" BorderStyle="none" Style="float: right;"
ToolTip="View Details.." />
</asp:LinkButton>
</div>
</div>
</asp:LinkButton>
<script language="javascript" type="text/javascript">
function DemosClick() {
TitleClick('<%= DemosExpand.ClientID %>');
}
</script>
</Header>
<Content>
<table cellpadding="0" cellspacing="0">
<tr>
<td>
<img src="images/content_top.gif" width="100%" alt=""/></td>
</tr>
<tr>
<td style="background-image: url(images/content_middle.gif); padding-left: 10px;
padding-right: 10px;">
<!-- Content -->
Below are a few demos demostrating Trimedia's ability to make your project come
to life. We are adding new demos regularly, so check back often:
<ul>
<asp:LinkButton ID="LinkButton18" runat="server" OnClientClick="doMovie('MedicalDemo.flv'); return false;"><li>Cardiology Patient Education Video</li></asp:LinkButton>
<asp:LinkButton ID="LinkButton19" runat="server" OnClientClick="doMovie('AnimDemo.flv'); return false;"><li>3D Animation Demostration</li></asp:LinkButton>
<asp:LinkButton ID="LinkButton20" runat="server" OnClientClick="doMovie('AudioDemo.flv', false); return false;"><li>Audio / Commercial Demo</li></asp:LinkButton>
</ul>
<atc:ModalPopupExtender ID="ModalPopupExtender1" runat="server" BackgroundCssClass="modalBackground"
OkControlID="OkButton" PopupControlID="VideoPanel" PopupDragHandleControlID="image2"
TargetControlID="LinkButton18">
</atc:ModalPopupExtender>
<atc:ModalPopupExtender ID="ModalPopupExtender2" runat="server" BackgroundCssClass="modalBackground"
OkControlID="OkButton" PopupControlID="VideoPanel" PopupDragHandleControlID="image2"
TargetControlID="LinkButton19">
</atc:ModalPopupExtender>
<atc:ModalPopupExtender ID="ModalPopupExtender3" runat="server" BackgroundCssClass="modalBackground"
OkControlID="OkButton" PopupControlID="VideoPanel" PopupDragHandleControlID="image2"
TargetControlID="LinkButton20">
</atc:ModalPopupExtender>
</td>
</tr>
<tr>
<td>
<img src="images/content_bottom.gif" width="100%" alt=""/>
</td>
</tr>
</table>
</Content>
</atc:AccordionPane>
I ran it through the Javascript debugger in FireFox and everything seems to setup properly on the ModalPopupBehaviour, but something in the way that the Accordion works somehow breaks the Onclick event on the LinkButton because it is never calls the ModalPopup code (but the code placed in the OnClientClick is called).