That sort of message would make me wonder whether malware is blocking what you access.

If that is the case you need to take advice on removal of malware.

You could try an alternative browser, but if it is an unresolved malware issue that could compound your problems.

What security software do you use ? And what is the OS XP?

I wonder is that message is actually telling you to close Firefox before starting the Norton installation procedure??

Norton typically wants to install a couple of Firefox extensions, and the manner in which they perform that extension installation probably requires that Firefox be closed.

In the case of Malware, the entire Norton domain would typically be blocked and the user couldn't even load a web page there, much less download anything. No personal experience with that though, just what I have read about some Malware blocking AV sites and anti-malware websites.