I do find the name – "containers" – a bit confusing. "Contextual identities" would seem like a better fit, because that's what it will be used for.
I am reading this name also on other places, surprised Mozilla sticked with "containers":
* https://blog.mozilla.org/tanvi/2016/06/16/contextual-identit...
* https://wiki.mozilla.org/Security/Contextual_Identity_Projec...
* https://wiki.mozilla.org/Security/Contextual_Identity_Projec...