Finally the problem is solved, and it has not to be with web2py. I am using jqm (Jquery Modal Plugin)
The sample code has
<div class="jqmWindow" id="photomodal">
<a href='#' class="jqmClose">{{=T("Close")}}</a>
<br />
<img class="photo jqmClose" src="" />
</div>
where the src attr for img is filled dynamically by the plugin modal.
Firefox was raising validation error because of that.
I used the w3c validator to solve this problem!
Thank you all
