This extension has a nice set of features, but suffers from too many problems to be usable for me:
- the search bar for tabs grabs the focus when creating a new tab
- the order of tab creation is changed: new sub tabs are created above the previous siblings
- closing a tab brings you to a seemingly random tab, not the next sibling
- Firefox operations are noticeably slowed
- it sometimes get confused, and the order of the tabs is not the same as in the top bar
- no drag and dropping tabs between windows

So, worth it if you really need the specific feature, avoid otherwise.