Thanks for the extension, I like this, just still have a small issue.
Opening another closed group while already in a group's tab, that group will be merged and disappear. I look forward to a new feature to enable/disable it.

[edit] Pressing the "1-second delay with cancel" shortcut while reopening the tab group seems to work. Maybe that helps :)

[edit] Thank you for your quick response. Unfortunately it still does not work for me, although I am using version 1.5.0. I am using Linux Mint Cinnemon with the Firefox-Version 138.0.1 by the way.
Just to be clear: I am talking about the drop down menu shown in this screenshot (in this case you would reopen "Ghacks") : www.ghacks.net/wp-content/uploads/2025/04/firefox-tab-groups-stable.png

Thank you for implementing this missing feature. Unfortunately I think it breaks the reopening of tab groups that were saved upon closing (accessed via the tab groups overview) by adding the tabs of the group into the group of the currently active tab instead of reopening them in the group they were saved in.
5 stars if fixed :)