Any reason why new tabs shouldn't create an entirely new, isolated container by default? And then allow the user to close tabs implicitly by performing a merge operation with some existing container (say, dragging with the mouse with a hotkey held down onto an existing tab group representing a single container)? And alternatively, "merging" with the empty container, which would retroactively implement incognito mode…
https://addons.mozilla.org/en-US/firefox/addon/containers-on...
Someone also made a theme which will color the temporary container windows a different color so you can easily differentiate them from the normal tabs/windows.
https://addons.mozilla.org/en-US/firefox/addon/containers-th...