Retainer от Roy Orbitson

This is an updated and improved version of https://addons.mozilla.org/en-US/firefox/addon/new-container-tab/

It provides a keyboard shortcut (Shift + Alt + C by default) to create a new tab that retains the current/last-used container.

You can edit the default shortcut under Add-ons and Themes > Extensions > ⚙️ > Manage Extension Shortcuts > Retainer. New tabs can be restricted to the last-used container in the extension's preferences panel under Add-ons and Themes > Extensions > Retainer > ⋯ > Preferences.