I am trying to find what someone typed into Firefox on a particular site?

I am not trying to get a password. Only information that would have been visible when typed in.

You would need a keystroke logger to capture what they are typing. I’m not sure of the legality of using one without them knowing. If it’s a personal friend or family member, it’s really just sneaky on your part. If you do it in a place of business without warning the person of the possibility first, there could be some legal issues there.

I word-for-word agree with squilky. A key logger logs all keystrokes that are typed during a session and can usually be run off a USB.
Example: http://www.actualspy.com/
I hope that it’s not for a malicious reason, so good luck!
Also, you could try asking your friend if the situation permits.