Group Tabs by Container од Yuki Suter

Group Tabs by Container keeps your tabs organised by folding each Firefox container into its own native tab group, matching the group's name and colour to the container it represents.

Unlike other container-sorting add-ons, it operates one window at a time and never moves a tab from one window to another. Each window is grouped independently, so dragging a container tab into a second window regroups it there instead of dragging it back. If you work across several windows, nothing gets scrambled.

It's built to be unobtrusive:

Pinned tabs are left pinned and untouched.
Ordinary (uncontained) tabs are left alone by default.
Groups you've made yourself aren't disturbed, it only manages its own container groups.

You can group on demand from the toolbar button (this window or all windows) or with a keyboard shortcut, and there's an optional switch to auto-group new tabs as you open them. Everything happens locally on your machine, the add-on collects and transmits no data of any kind.

Requires Firefox 139 or later (for the tab groups API)