Firefox containers keep cookies and logins separate per context. The extension can assign a proxy to each container, so everything opened in that container goes through that proxy regardless of which mode you are in.
Assign containers on the Containers tab in the extension’s options.
This is Firefox only. Chrome and Edge expose no container information to extensions, so a request’s container cannot be detected there — and rather than carry a tab whose only content would be that explanation, those builds ship no Containers tab at all.
Precedence
A request is resolved in this order, and the first match wins:
Bypass list
Anything on the bypass list goes direct. Localhost and private ranges are always bypassed, including from a container. Container assignment
If the request comes from a container you have assigned, it uses that container’s proxy. This beats both the global mode and your rules.
Mode and rules
Everything else follows the current mode: Direct, a single profile, Auto with rules, or System. Unassigned containers
A container with no assignment behaves like any other tab and follows the mode and rules.
Deleted profiles
If you delete a profile that a container was assigned to, the assignment is dropped rather than left pointing at nothing. The container returns to following the mode and rules. 
