I FOUND IT!!
Now IMPORT can open!!
THANKS A LOT!!!

Awesome add on ! Seems to be a great successor for tabkit 2 !

Some ideas and remarks :

Color tabs when a child tab is opened , I mean that each tree could be colored in a different color : it's very useful ! (a click on a link opens a new tab in the same color of the current parent tab, the main parent tab is colored only when a child tab is opened) .

Highlight tabs in a special color for links opened in a new tab but the tab haven't been opened/clicked/viewed.

You could also add a checkbox for "mousewheel scroll on the last tab allow to return to the first one".

Shift + mouse scrool to switch between groups

A keyboard shortcut to close a tree could be nice too ;)

Add a tutorial on the add-on web page to explain how to remove the original horizontal tab bar from firefox.

Thanks for the work on this add-on !

The only extension seamless combining tree style, folders and groups. Also theme customization is amazing.

Great add-on. I'm still not a big fan of horizontal tabs but I'm sticking with it until a real a «Tab Groups» alternative comes about. The ones that exists have many problems that need fixing.

Of the "tree tab" add-ons I've tried this one has the most feature sets I'm interested in. Most important >Tab Grouping... Tree hierarchy, Tab Unloading, Searching, Sorting are just really nice features. The latest version can now 'hide' tabs when you switch groups without closing them so now it even more useful. You can switch tab groups and then hide the sidebar if your strapped for screen real-estate.

After «Tab Groups» etc. died because of WebExtensions I had to find some good alternative and settled with this one. It is so feature rich that I consider contributing to this great Tab Manager

Cannot live without it, but just today it started losing the grouping of 200+ tabs every time i restart firefox. I can provide you more details about the problem if you want, I need it to get solved because grouping it's the main reason i use this extension.
Thank you

Liking this so far, lots of nice features, but haven't tried them all yet. The groups feature is a welcome addition. I could say this is an addon that could replace a bunch of other ones I have (session saver, tab groups) if the tab group import and expert, and the tab session import and export worked for me, at least it doesn't right now on my Mac, High Sierra. If it works on Windows that'll do me as that's my main power user machine anyway.

Issue I am having on the Mac is that the export saves it as a csv, and importing looks for *.tt_group or *tt_session - renaming the extension from csv to the correct one it's looking for, or just trying to load in the csv - both methods result in nothing happening.

Also the search feature would be better if it just showed the matching tabs rather than highlighting them. But personally not a feature I can see myself using much as I'd just search using the address bar.

Good effort though, switched from Tree Style Tabs as half the time the tab side pane would just load endlessly and never complete.

EDIT- the import/export works well on Windows, but it crashes my Firefox A LOT :( weird, as I don't have this problem on my Mac. It will hog maybe around 50-60% of my CPU :/ disabled for now, but I really want to re-enable it later down the line, I really want this addon to do well.

Much less buggy than Tree Style Tabs. Also I love the search tabs feature and it seems to be more lightweight and more responsive than TST

The add on is really great and an amazing productivity boost. Unfortunately, just now I lost all my tabs in the addons grouping functionality - one day some tabs could not be clicked on to switch to it so I restarted and all was lost apart from the ungrouped group. Not sure what happened.

Excellent, but somewhat bloated if you only want vertical tab without the tree hierarchy. Few issues here and there.

I've had to downgrade my review because of one tiny but huge thing. For a very strange reason, the author has chosen 'F2' to show/hide the panel. This is not a good choice as F2 is universally used for 'edit'. So i've had to disable/remove this add-on because it kept showing/hiding and stealing the focus through the use of this shortcut. My example is through using Google Docs.

I humbly request he changes the default to something that isn't so commonly used and perhaps a two-key combo (Win+F2?). On top of changing the default, i suggest making it customisable. Thanks.

Great addon, the save session (and even group) function is indispensable for me. Every time I launch FF, I need certain tabs open by default. Saving my sessions through this addon lets me load up what I was last working on, and merge in any default tabs that I need. Very good addon, I highly recommend if you want to save your tabs and quickly get back to work. One thing though! This is the only bug I have encountered; The tabs sometimes will stop responding to clicks, and sometimes its not just selecting tabs but specifically moving them (one or the other, though). Simply closing tree tabs, and then clicking on the icon to get it back up fixes the problem. A little annoying but worth the effort since this addon saves me so much time.

Kroppy, you've done all us FF users a great service - THANK YOU! As a long-time user of Tab Folders (and all the various add-ons that worked up until WebExtensions killed all the great add-ons I'd used for years) I was in limbo until I stumbled on TT. You've given us all a way to organize the dozens of tabs and access them easily. And you listened to all of us begging for folders and groups and you built that in, as well.

Now I just need an idiot-level tutorial to figure out how to USE all these marvelous tools...

Has a very annoying feature; once in a while it loses tree structure when ff gets restarted.

Great add-on, cannot live w/o it - nicely replaced the old tab groups manager & helper. Feature request - is there a way to allow right click the tab and let me save bookmark?

EDIT: DON'T INSTALL TAB SESSION MANAGER.

Kroppy, thank you very much for the tip. After removing TAB SESSION MANAGER, your extension now works perfectly. Maybe adding that information to the description and/or work with the owner of that extension to make both compatible.

This is the most wonderful extension for Firefox, period.

First impressions: I'm just now testing Tree Tabs, as a long-time Tree Style Tab user who's followed Tree Tabs from the first Vivaldi forum posts. It has some very nice features out of the box. I love the groups, unloading, and session backups, as well as the auto-correcting tab order for keyboard shortcuts.
Additional thoughts: So far, pretty awesome. Literally replaces three extensions, and very nearly three more, at least. Groups, tree-hierarchical tabs, and tab search are fully replaced, and it almost replaces a session manager, tab-unload manager, and an undo-close menu.
If wishes were... : It'd be helpful to have the session feature save automatically every x minutes and allow selection of a session to load. Tab-unloading would be more useful if it could be automated on a timed basis with a whitelist. Undo-close would be nifty if it had a list of the last few closed tabs, but it works fine as-is.

Bugs: None found yet?
Noteworthy: The audio icon only appears on that specific tab, even if it is within a collapsed tree. This can be a bother if more than one tree or branch of possibly-guilty tabs exists. It might be better to display the icon on the nearest visible parent tab, if the playing tab is hidden, as well as on the group title, to aid in locating an offending tab, especially in cases of sneaky, ill-behaved ads, but also for those who (over)(ab)use YouTube and the like.
Edit: Found the sidebar unresponsive at times. The problem was predictably resolved when I disabled my session manager and a tab-unloading addon. Seems that many (most?) things that interact with tabs directly may interfere with Tree Tabs.