Container cookie duplicator от Koen van Ingen

What does it do?
This plugin automatically synchronizes cookies from the 'main' Firefox container to all the other configured Firefox containers. This can be useful for instance to copy your existing login sessions to other containers.

How do I use it?
On the settings page, you can specify domains you want to copy. After configuring your desired domains there, this plugin will automatically synchronize all the cookies of those domains from the default container to the other containers. It works only 'one-way': it copies cookies from the default container to the other containers, but it doesn't copy 'back' to prevent cookie conflicts.