Session Manager
… thanks, Mozilla bug 1322060 - Add WE API to provide functions of SessionStore.setWindowValue and SessionStore.setTabValue also found via the forum links in MozDev bug 26384 (in post 21 above) 
Showcase
There’s significant potential for the cross-container search feature of Taborama to partially fill the gap that arises from incompatibility of Firefox 55 with Showcase.
Whilst Taborama is listed as working with Firefox 57.0a1 and later, users of 55.x may find the search feature usable. I’ll be particularly interested to know whether search is reliable where 57.x has more than five hundred tabs in a session.
Tab Groups
Generally, I don’t treat container-/context-based extensions (such as Taborama) as potentially full-featured successors to Quicksaver’s Tab Groups.
Issues such as this may be exemplary:
– arising from the underlying emphasis on containment without movement.
Tree Tabs … maybe.