With First Party Isolation is place, containers now add up very little to your privacy to be honest. They are mostly helpful in convenient compartmentalization of your browsing activities without actually having two different browsers.

Firefox is already very privacy focused, and its default settings are pretty good.

Partially incorrect. There is unnecessary telemetry that you would prefer to get rid of, for an example there is a setting for extensions recommendation as you browse. Also, probably because of their deal with Google, Firefox defaults to Google’s location services even though Mozilla has its own. You may want to change that as well for better privacy. I am only citing a handful few examples, there is more for you to dig in. uBO is a must have with right set of filters enabled according to your own privacy threat vectors. There is a reason hardening is a common practice among Firefox users.