New Container Tab by Karim Rahal

Opening a new tab in a Firefox container is hell—you have to either hard press the new tab button or remember obscure, limited shortcuts. New Container Tab changes that: you use the shortcut Ctrl+Shift+Comma, search for a container name, and press Enter. Furthermore, the container search box auto-suggests, so speed is guaranteed! You can also enable the option to 'auto-open' tabs once a match is found.

https://github.com/KarimPwnz/firefox-new-container-tab