Hi

I think this is the add-on that you are looking for:

https://addons.mozilla.org/en-GB/firefox/addon/multi-account-containers/

This is the correct extension but I already have it downloaded it just will not work for me as you can see in my screen-shot it is downloaded but does not show up with the rest of my add-ons.

Are you using Private Browsing? I'm asking because according to your screenshot you did not grant the Multi-Accounts Container extension access in Private Browsing, meaning that you won't be able to use it if you are in Private Browsing.