They don't have to use it, but this webextension gives the ability to only keep and/or remove certain site data or specific cookies - the main Firefox does not do that (they do all or nothing). This webextension also allows per container customization.

Please add any requests to the GitHub issues/feature request area.

We currently do not keep track of when the site was last visited though, so that may be harder to implement (i.e. which

Your closest thing is to add a wildcard '*' entry to the greylist/restartlist and enable 'clean open tabs on restart' option, then it would run every time you restart your browser. Probably not as periodic as you want though.

This web extension requires some configuration before enabling it to ensure the cookies and other browsing site data you want to keep is configured. All other non-configured items are removed, as that is the primary function of this web extension. If you want to only delete cookies from private browsing mode, add a wildcard '*' whitelist entry for all other containers.

While we are able to delete a few items from Private Windows, you will need to ensure you allow this web extension to operate in Private Browsing mode.