TabGroupSync od Hannes Coulembier

Source code: https://github.com/HannesCoulembier/TabGroupSync

All linked groups are synced across your devices (if you have syncing bookmarks enabled in your firefox settings).

Click on an untracked group to save and link it. Click on a saved group to open and link it.
Click on the copy icon to open a saved group without linking it and on the close icon to remove the group from your saved groups list.

If you have an untracked group and want to save it into an existing saved group, you can click on the circle of the untracked group and then select which saved group you want to overwrite.

If you have restore tabs enabled when restarting firefox. The extension will try to link the restored tabs to your saved tabs, so you don't have to link them manually by overwriting the saved ones with the ones in your browser.