Clearing service workers is sadly not possible on domain change. That is a limitation in the Web-Extension interface. Beware of the term localstorage. Firefox shows items as "storage" in the settings even if they are "indexeddb" for example. What third-party cookie setting have you used? Try visiting http://github.com/lusito/forget-me-not for better support about issues.

Hi, sorry about your bad experience. Flower being gray means, that you are using a private window. If the icon is not showing, maybe it got hidden in the overflow menu (the arrows). Try selecting "customize" from the firefox menu.

It doesn't require container tabs. It just needs the permission in case you use container tabs so that it can perform the cleanup properly. If you don't use container tabs, Forget Me Not will not use them either.