Firewall and SSL working with Web Form info

I have an Intranet web site that has a form where user enters social
security number and other sensitive information.
The Web server that processes the info is behind a Firewall.

I want to register an SSL certificate on the Web Server to protect the
transmission of information from the Web Form to the Server. Please
advise if this is necessary since the Web Form is an Intranet Site and
the web server is behind a Firewall?

I assume I need SSL to protect the Web Form information from the
Client workstation until it gets to the Firewall and then web server
or how does this process work with protecting the information??