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

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

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...

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.

I use this as a replacement for the discarded panorama feature. Tree structure proves to be handy or even extremely useful. A spot-on use of the lateral panel, making for a perfect integration.

Still sketchy in nightly (currently 61) with new windows popping up when dragging tabs. Otherwise, quite stable and reliable. [Edit] As of Nightly 61, no tab dragging issues after several hours testing in 61. So I was wrong, nothing sketdchy about it :). Awesome!

I would still browse pre-quantum if it wasn't for this. THanks indeed.

Seems to break the lateral panel or rather lock it down. I installed an experimental feature (notes) which uses the side panel as well. NO dice. I suppose it's worth looking into it here :
https://testpilot.firefox.com/experiments

This extension makes me say "WOW!" It has so much functions to control tens and hundreds of tabs.. Definetely more fucntional than Tree Style Tabs plugin which I used before, I going to use this plugin instead.

Thank you for your efforts in manufacturing this software, it's useful and beautiful. I have already recommended this software to my friend. I have only two questions, both of them are about the session.

(1)When I restart the Firefox, and click "History -> Restore previous history" in Firefox menu, tabs come back but without folders and groups, I have to import my previous session backup to solve this problem, using the "Import and merge session" option.I hope this is an automatic operation.

(2)When I start the Firefox, click the add-on's menu "Import session", and select my session backup, and horrible things have appeared, Firefox start loading the tab one by one, because amount of my tabs is huge(about 150), so it cost me 20 mins to load, and finally used 7G memory.

Can you solve these 2 problems? My Firefox's version is "Firefox Quantum 58.0.2(64-bit)". Looking forward to your reply! Thank you!

Make Tree Style Tabs looks overrated. =)

Hey, a suggestion:
What about a new config to bind groups to containers?
Then opening a new tab on that group will always open a tab of the chosen container.

This is fantastic! The features are so much better than Tree Style Tab! Why doesn't this appear as a related extension or higher in the search rankings!?