This seems like a reimplementation of Profiles, that you can use with `firefox -P`.
The main issue here is one of UI. If you click a link in a PDF, what profile should the link be opened in?
My take is that if you have multiple firefox instances it should ask you, otherwise it should just open on the only one open.
The idea here is to be lighter-weight than profiles, or the similar feature in Chrome. I've got three different containers going right now, side-by-side in one browser window. In addition to the per-tab basis, providing separation within a profile also means you don't have to set your preferences, addons, etc. independently each time.
For example, one of the major use cases for this sort of separation is being able to use multiple accounts on a site like Twitter or Facebook (which don't support multi-account) or Gmail (which does, but it's a bit rough). There's no reason I should have to have separate windows or separate addons for each of those accounts.
Just to be clear, this is NOT just a simple UI version of profiles. Containers only separate Cookies, localStorage, indexedDB, HTTP data cache, Image Cache (bug in progress), and other areas supported by originAttributes (See https://wiki.mozilla.org/Security/Contextual_Identity_Projec...)
I.e., Containers do NOT protect against some lower-level privacy vulnerabilities like plugin enumeration or history-sniffing that are mitigated more by using entirely separate profiles.
The main issue here is one of UI. If you click a link in a PDF, what profile should the link be opened in? My take is that if you have multiple firefox instances it should ask you, otherwise it should just open on the only one open.