I don't know why 'Tree Style Tabs" is recommended by FF and this isn't. This addon leaves TST for dead and is by far the best tab manager I have come across in a thorough search and test.
It is so superb that a manual of all the things it can do would be helpful! The basic sidebar setup with drag-and-drop can be instantly used by new users, but I am still not sure if I have found and figured out how to use all the other brilliant parts of this addon.
It has multiple backup options- customisable session autosave, EXTERNAL save a group or whole session, import between profiles, reload separately or merge. Of all the features in this addon missing in other tab organisers, this is the most important. (You can use Onetab to create tab indexes for saved groups and sessions - save, restore in new window, Onetab.)
I use it combined with profiles as a task management system, managing thousands of tabs. Profiles for areas of work (to keep memory manageable), Custom named groups (thin vertical tabs at the left, can be hibernated when not in use) for categories, custom named folders in those groups to divide tasks (and comment on them!), and finally trees for subpages so I know where I am.
Lots of useful options for groups, folders and trees-- collapse, suspend, reload, bookmark, hibernate, search, duplicate tabs. Customise where different types of new tabs open.
Highly customisable UI: tab height and indent (Yay! fit heaps in sidebar, minimises scrolling), colours for hover, active, selected, search and combinations of them, 1-click save customisations.
AND it imports tabs unloaded - so memory and custom tab titles are preserved (until the tabs are loaded).
The only major thing missing is container support! Only 1 tabs manager has this (Sidebery, but it's structure doesn't work for me), it is SO needed. (Though merge does keep the containers of the previous tabs, other imported tabs have to be reopened in new container which loses their place in the tree structure.)
A few bugs in a some of the advanced features, a headache but those features don't even exist in other managers. A very good issues site and responsive dev who keeps working through them.