It's exactly what I've been looking for, but I have a few gripes about it:

1. Moving existing tabs to containers is a real pain (new users). The only way I know is to Right click tab > Reopen in container, but this produces a duplicate and then I have to delete the old tab.
2. It's very slow to toggle containers on/off and you can't see which ones are currently switched on. it could do with a simple toggle method which would grey out the group or show a hidden icon next to it. Personally I would like to use 1 group at a time and leave the others off.
3. Very limited list of colours and icons for containers. It needs basics such as music, films, news, coding, videos etc.
4. I've moved the icon to replace the new tab button in the toolbar using Customize. Could the CTRL+T shortcut be linked with the addon as well?
5. After adding loads of tabs using above method 1 of my groups is totally empty, others seem suspiciously empty (1-2 tabs only), I have lost quite a few tabs by using this.
6. Not compatible with "group speed dial" for opening new tabs
7. When reloading the browser tabs fall out of containers and they become empty, defect?