Not completely. Incognito mode is still useful so that when your boss sits next to you, and you start typing words in the browser/search bar, it doesn't offer "interesting" autocompletion based on browsing history.
I think that's exactly the parent's point. Incognito mode is useful despite being essentially "security theatre". The same way that requiring a master password to view stored passwords is useful.
Yes, I agree that master passwords are badly implemented in Firefox.
It should be on by default — like Safari where it uses your keychain password when attempting to display a plain text stored password. Chrome and Firefox could easily do this too (they do use the Keychain to fetch passwords anyway.)
