Philippe,
Since that link is using a Javascript popup, the Joomla system plugins aren't being triggered on that content. You'll need to change from using the {JFBCLogin} tag to using either a text or image link. Below is an example:
<a href="javascript:void(0)" onclick="jfbc.login.login_custom();"><img src="link-to-image.jpg" /></a>
Put that in there, and it should work find (either change the image link or just replace that with text.
Hope that helps, but if that's more confusing or you need more assistance, just let us know!
Alex
{JFBCLogin} not working
