Lets assume you have an web app with some sort of login system, and you are testing things on a local web server bound to "localhost" and you want to test multiple logged in users at the same time.
We start by creating multiple "domains" pointing to the localhost via the /ets/hosts file:
127.0.0.1 localhost1 127.0.0.1 localhost2 127.0.0.1 localhost3 127.0.0.1 localhost4 127.0.0.1 localhost5 127.0.0.1 localhost6 127.0.0.1 localhost7 127.0.0.1 localhost8 127.0.0.1 localhost9
Because the session and local storage is instanced per domain name (or even the cookies for the old school) using the localhostX will each be isolated.
Then you fight with your web server that definitely will not like requesting http with something different than localhost, but you can win.
And then you fight with the OAuth callback redirect url and... you lose the fight... and you go back to the firefox containers.
done_