Great addon! I love it! But, since a few weeks I got always this message, when I open the browser:

Simple Tab Groups addon conflicts with the following enabled extensions:

Enabled:
Tab Session Manager

To avoid losing tabs or their location in windows and groups, it is strongly recommended to turn off or remove enabled extensions that conflict with the Simple Tab Groups addon. You can do this on the addons settings page "about:addons" or by pressing the key combination Ctrl+Shift+A

???