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 it can take a lot of time to find and figure 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, merge sessions. Of all the features in this addon missing in other tab organisers, this is the most important. MISSING in tree style tabs. TT's bookmark feature creates tab indexes for saved groups and sessions, structured by group, folder and trees. (Or Onetab - save, restore in new window, Onetab.)

Can use TT search in each window to select and drag tabs by title or url between groups, folders and windows. (Use Conex for cross-window searches by container.)

I use TT 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. (Bookmark imported groups or sessions before loading to retrieve custom tab titles with Tab ReTitle addon.)

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 - you need to import in a new window, bookmark the import, save the session, reopen the imported tabs in containers, manually rename the custom titles, then merge the saved session and hope it works.)

A few bugs in a some of the advanced features, a headache but those features don't even exist in other managers.

29/9/20: I am marking this down to 4 because I am SO SICK of spending days manually fixing lost/moved new tabs/trees/groups/sometimes entire session structures. Occasionally, randomly. neither restore nor merge of backups works and the dev has done NOTHING in over a year to fix this bug, or to integrate containers, and STOPPED someone else who tried to. I'm going to mark down 1 star each time I get mad enough to visit.

Of the minuses:
-No general tab search across all groups.
-Auto sort by template does not work
-Skinny setting hotkeys

Great and essential Add-on ... better than Tree Style Tabs. However, Automatic Tab Assignments has stopped working (even tried on a fresh Firefox install). This was a big help keeping tab-sprawl at bay, as now I have to manually keep moving my tabs around. I'll keep using it as long as it's mostly working, but I'm sad that development seems to have stalled.

After some months of treetabs missing in the sidebar, it has suddenly reappeared, with all 300+ tabs still in order!
I'm so happy about this uber helpful addon getting back on track!
My donation is meant as a big "Thank You!" as well as an incentive to keep on caring about this georgeous baby.

Great bu buggy...
the tabs all appear in the same group (loss of structure).
Plus various bugs monthn annoying.
And sometimes all the tabs disappear. This morning, I lost everything, it's not acceptable: I give up to find something more reliable.

Easily my favourite extension that allows side tabs, as it allows labelling and grouping of tabs into groups.

The problem I describe below the fold is easily fixed by the declopers' suggested workaround, and hopefully will be fixed soon.

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The extension constantly flashes the entire tab sidebar section white every 2 seconds or so - this only started happening today, but I'm not sure why.

Here's a gif of the behaviour - https://gph.is/g/4gpQ7WE

After restarting Firefox, all tabs previously sorted in many groups are in Ungrouped. All informations about tabs in groups are lost. Sorting tabs into groups has already been specified for add-on. There is no situation for the add-on to lose this information. Add-on has poor session management and is unreliable. It is very frustrating for the user to reorder all tabs into groups again and again...

After some time, about 80% of the pages on other tabs were lost. The plugin has **absolutely unreliable** status information management.

This extension seems to be awesome but unfortunately I can't make it work.

All tabs come back to "Ungrouped" after restarting Firefox and even saving the session previsouly doesn't restore them ok.

The message "Session did not match and will not save! Click here to reload" is appearing too. I'm using Firefox restore session, but checking/unchecking it doesn't seem to help either.

Such a shame, this is everything a tab freak could want (groups, folders, tree structure, load/unload) but without a really strong session management, it makes it hard to trust. There's nothing worst than sorting lots of tabs into groups and folders and tree structures to find out later that you've lost all of that back to a flat structure.

Hope the developer can fix this, it really could be an essential piece of software for those of us who work with lots and lots of tabs.

EDIT: It seems if I check on the "Discard tree structure after browser's restart..." option, click on the reload tree message (and then uncheck the "Discard tree structure after browser's restart..." option again) everything works ok.
Haven't had a single crash or problem since, in any of my profiles (and I have quite a few). So there may be some type of incompatibility with my older extension (tree style tabs).

Awesome extension... I'm donating to the author. Hope he can keep up the good work!!!

Sometimes there is a bug where all the tabs are extinguished.