I use this as a replacement for grouped tabs in Firefox 57. Actually it's better than grouped tabs since you can't have nested groups. With this Add-on you can.
But the Add-on really needs an option to NOT open new tabs as a child tab when I middle click on a link. Opening a link from a website often but not always says something about the hierarchical relationship between those tabs. When I click on a link in a list of search results that makes sense. When I click on a "see also ..." link at the bottom of the page of some documentation or on a link to related videos on YouTube it's just nasty that those new tabs always become child tabs of the current one.