Sorry, I don't understand what you are asking about. The Sync Key is the encryption key, it needs to be plain alpha/numeric text so it can be entered when needed to connect to the sync server or when you add a new device.

Or are you referring to it being shown in the Password Manager > Saved Passwords window?

Yes I am referring to it being shown in the Password Manager > Saved Passwords window, anybody can see that key, is that a good idea?

Are you using a Master Password?

• https://support.mozilla.com/kb/Protecting+stored+passwords+using+a+master+password

No, I click on "tools", "options", "manage account" and on the tab "sync" I click on "manage account" where I click on "my sync key" and there the number shows up, which could be seen if someone other than myself is in this house or when the computer is taken away, or stolen.

They would still need the password for the Sync account to get access to the data on the sync server and if you use a (strong) master password then they wont be able to get that password.