The solution is an extension called Temporary Containers. You have to turn it to "Automatic mode", and fiddle a bit with the settings, but I have it set up so

The solution is an extension called Temporary Containers. You have to turn it to "Automatic mode", and fiddle a bit with the settings, but I have it set up so that every time I open a new tab it is a fresh isolated session, and will be destroyed when the tab is closed. When I need to be logged in, I use the regular containers in Firefox for each service, or a context like "work", instead of the temporary containers.