I'm going to answer my own question.

Maybe this can go to the firebird wiki since what's there is outdated
and does not work with Tomcat 5.5.

This sets up a pool managed by tomcat in some standard way I'm not too
familiar with yet, but you can forget about everything else.

Place firebirdsql-full.jar in ${CATALINA_HOME}/common/lib

In the application's context.xml

<?xml version="1.0" encoding="UTF-8"?>
<Context path="/PooledWebapp" reloadable="false">
<Resource auth="Container"
driverClassName="org.firebirdsql.jdbc.FBDriver"
maxActive="100" maxIdle="30" maxWait="100"
name="jdbc/dbTest"
password="masterkey"
type="javax.sql.DataSource"

url="jdbc:firebirdsql://localhost:3050/c:\fede\test.fdb?lc_ctype=UNICODE_FSS"
username="SYSDBA"/>
</Context>

In the application's web.xml
<resource-ref>
<description>DB Connection</description>
<res-ref-name>jdbc/dbTest</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
</resource-ref>

To do sql inside an index.jsp

<%@ taglib uri="http://java.sun.com/jsp/jstl/sql" prefix="sql" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>

<sql:query var="rs" dataSource="jdbc/dbTest">
select id, nombre, telefono from persona
</sql:query>

<c:forEach var="row" items="${rs.rows}">
Id ${row.id}<br/>
Nombre ${row.nombre}<br/>
Tel: ${row.telefono}<br/>
</c:forEach>

To get a connection inside a java class
Context initContext = new InitialContext();
Context envContext = (Context)initContext.lookup("java:/comp/env");
DataSource ds = (DataSource)envContext.lookup("jdbc/dbTest");
Connection conn = ds.getConnection();