This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
404 error means its problem with path of your .class file and another thing is you should give the folder name before firstServlet.You have to copy the complete path of the .class file in the webapps folder in your tomcat. And paste it in the web.xml file.
Everything was correct expect in your 'hello.jsp' form, just you remove the slash in front of <method="FirstServlet"> now it will work perfectly. Because in web.xml the urlpattern will take default "/", so it is not nessassary to put / in form action.