You can read Shibboleth SAML attributes sent by the IdP using Request.ServerVariables object:
string server = Request.ServerVariables["HTTP_FIRSTNAME"];
See this if you want to list and print all the attributes in session.
Remember to configure Shibboleth attribute-map.xml to handle the custom attributes your IdP may send:
<Attribute name="firstname" id="firstname" nameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:unspecified">
<AttributeDecoder xsi:type="StringAttributeDecoder"/>
</Attribute>