There is a support article that might be relevant to this: How to fix preferences that won't save. Is anything there helpful?

That helped jscher2000. Within the article, it mentions editing the users.js file manually so this is what I did.

The line in question was:

user_pref("browser.urlbar.autocomplete.enabled", false);

Of course changed it from true to false and saved it.

Seems to be ok now.

Thanks for the tip.

The user.js file is read every time Firefox is started, so settings stored in user.js will override settings that have been made in the previous session (e.g. via about:config or otherwise in the user interface) and were saved in prefs.js.
You can only change them on the about:config page or via the user interface for the current session and not permanently, so be cautious with using the user.js file to set prefs as you may forget about this.
In such cases you need to make the change in user.js or remove such a user_pref() line to make them work across sessions.