The main idea is good when I want to logg in for more than one acount on the same site, but...
when I load a site in a container, the first cookie for that site will also remind outside that container. So if I use a cookie autodelete add-on and quit the loaded site in a container, that cookie will still remind in Firefox. Then I have to delete the cookie manually to get rid of it, and THAT IS A VERY VERY BAD IDEA!

For me it's better to load a news site in an ordinary tab so the cookies can be deleted automaticly at a choosen timelaps, and thus preventing cross-links towards that cookie.