All the incognito browser windows share the same “session” in Firefox. So say you open an Incognito window to browse Facebook or something, then you open another Incognito window, this new incognito window is linked to the previous incognito window, meaning you are logged into Facebook at that new Incognito window as well. This is because, as I explained before, all the incognito windows share the same “session”
The only way to clear incognito window is to close ALL of them and then create a new incognito window. You dont have to close the main non incognito Firefox window though, just close all the incognito windows. Then open a new one, now your previous session is destroyed and you are new again.
You may know it but its not that common knowledge as it should have been
You can move tabs between windows, so either you need to go with each tab is a separate container, or all private mode windows share a container.
Perhaps it makes sense to have multiple private browsing windows and label them to make it obvious? Or just eliminate private mode and just use containers for it with a setting for each container for which addons work with that container.
All of that seems complicated, so I prefer to just keep it simple. I use containers to separate concerns/accounts, and private browsing if I want to ensure browsing history isn’t tracked (esp. useful when searching for Christmas/birthday gifts).