"Cookie AutoDelete"[1] extension might be what you want. It flushes cookies after you close all tabs for a site. There's an allowlist of sites for which to retain cookies.
It's nice when paired with the "I don't care about cookies"[2] extension which auto-accepts cookie requests and makes sure you never see any cookie permission dialogs.
On Firefox you can select “Delete cookies and site data when Firefox is closed” under the browser privacy and security settings. You white list a bunch of sites you want to stay logged into using the “Manage Exceptions” button. No need for a plug-in. For all other sites you can use Firefox’s remember password feature.
Then, instead of using “I don’t care about cookies” you can simply tick all the boxes in the “Annoyances” section of uBlock Origin. That way you only need one plugin and not three.
That does assume that one closes Firefox, however. While Cookie Autodelete will remove cookies for a site within a few seconds† once the last tab is closed, and correctly handles tabs in containers.
†in case you're redirecting briefly for SSO or similar.
I found that having cookies actively deleted like that often crashes websites inadvertently and it ended up being a pain to manage. Allowing a website to track you for a day or so is not the same as being tracked for months. I believe that it is not nearly as useful to marketers. Closing and reopening my browser after a heavy web session is not a big price for me to pay. I do it to free up resources on my machine anyway.
Sites I log in to I don't clear. Other sites _shouldn't_ be able to tell the difference between deleting cookies and being a fresh visitor. This brings a new problem, in the form of cookie and GDPR banners. But there's an extension for that, too: https://www.i-dont-care-about-cookies.eu/.
I don't recommend it as a comprehensive solution because it misses some stuff, such as indexeddb and service workers. For maximum isolation you'd want temporary containers.
>The API for browsing data cleaning doesn't appear to support Firefox Containers. So if cookies from one container are cleared, then all of that site's data from all containers are deleted regardless of rules.
It's nice when paired with the "I don't care about cookies"[2] extension which auto-accepts cookie requests and makes sure you never see any cookie permission dialogs.
1: https://github.com/Cookie-AutoDelete/Cookie-AutoDelete
2: https://www.i-dont-care-about-cookies.eu/