First to say, I would like to give TST 5 stars, but with
a) Missing Search Function (more to that later)
b) Missing Savestate Function (more to that later also)
it cannot have more than three.
Unlike User Coolvibe, I have no problems with TST for more than a year. I have always about 900 - 1000 Tabs open. My OS is Win10 x64 with 16G of RAM, and pagefile is off. After a restart, it takes about 10 seconds to initialize TST icons, but with that much tabs it is ok. I would recommend to anyone who works like me to run process explorer to look at the memory consumption. If it reaches the maximum, which is easy in Firefox by changing between Tabs, FF becomes instable, and Tabs will begin to crash, maybe due to my deactivated pagefile.
I think that FF needs a lot more memory with TST, and maybe user Coolvibe is on Win32, which isn't really suitable anymore for some applications, FF included, because the devs do not care very much. I have on another machine a 32-Bit FF on a 64-Bit Win10, and it isn't as stable as the 64-Bit-FF, so maybe Coolvibes' problems are from there.
To paragraph a)
That is a very arrogant behavior of the TST devs to say on their page THAT REQUESTS FOR A SEARCH FUNCTION ARE IGNORED. Why ? The Opera/Chrome Sidewise has this. I am eager to know why a dev spends dozens of hours for a sophisticated program like TST, and refuses the most useful and simple functionality EVERYONE needs. I know people who are constantly using 2000 - 3000 Tabs and would be VERY HAPPY to have a simple search button. And NOT by installing ANOTHER addon that needs permissions to this and that.
To paragraph b)
A SAVESTATE Function would be very helpful in case of a heavy FF crash (that happens sometimes) or migration. Where tree informations are lost together with all open tabs. I use Tab Session Manager, but sadly, it doesn't save the TST tree informations. At least it didn't the last time, so my restored tree was completely flat, and I had to spend an hour or two to make new indentations (remember, 1000 tabs...).
Conclusion:
TST has a lot of useful functions, it grew from nearly crappy to very stable and relieable during a short period of time, but the focus went off a bit from real usability to functionality overload. I couldn't even find a SIMPLE function to give the active tab a special color. Maybe I'm too dumb or too impatient. But there are even several addons that are extending TST, and they do not fulfill the aforementioned search and savestate functions.

2-5-20 Review update:
I agree with Korkenwine review, the lack of Tabs Search options is a major missing option. Like many, when doing extensive online researching or shopping, I can easily have over 1K - 2K Tabs open, can get very hard to navigate in TST, do not always have time to go thru & clear out unwanted research tabs daily, that is why TST would be best to stay on top, to help quickly save multi list of ever growing tabs for later.
Again, TST is problaly the best (All in one) extension have found in last few years,but after so much troubleshooting, just cannot get TST to work consistently.
I am also using a fairly new Windows 10 64Bit Acer laptop, (not a 32Bit). I have ( 16GB RAM 1TB STORAGE), so I doubt if my PC hardware is the issue, as even most of the outdated Tabs extensions tested work, but have disabled them, due to lack of addtl options or updates. So my search, (would gladly Pay for even) for the closest TST addon & Full Tabs Mgr continues, until it gets worked out. Cheers!
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Great concept, after trying hundreds of addons, TST has to be one of the top extensions , with most useful features if have many Tabs, & need in user friendly vertical drop down view, for quick navigation & saving, So we do not have to keep installing & removing several outdated junk extensions , that only do a few basic Tab management options in Windows 10 PCs.
**Really wish Firefox & other Top browsers, would create a built-in browser extension like Tree style tabs , with most useful features if have many Tabs need in user friendly vertical drop down view, for quick navigation & desired selection of Multi Tabs saving & bookmarking etc.

Even though Tree style Tabs is a great concept, but like others have said, it just has far too many
Bugs & issues , so cannot even use it hardly at this time.
Like many, I also Used to use the older Tree style Tabs version until it, like most addons became outdated, & full of issues.
Was very excited when this new version was released recently, with many more options, but sadly, have still not been able to get it to work correctly last few most current Firefox versions.
TST loads all of my few hundred Tabs in vertical TST Sidebar, but that’s it, cannot search thru, or scroll
Thru more only 12 tabs out of a few hundred, TST doe nothing, tree style keeps freezing up.
After spending days & hours scouring thru all the Tree style Tabs FAQs, & support forums pages,have always had bug reports enabled, also keep restarting cache, unchecking & rechecking all options, many times, to no avail.
Like many, just do not have nay more time or energy to diagnose why TST addon not working still.
***Would be great (if TST Developers put in a Built-in Errors & Bugs troubleshooter & Fix option) so we do not have To keep trying to figure all TST's issues & bugs with each Browser version update.

Again great concept, could be the one of the Best extensions ever, if developers could
Work out all the ongoing Sidebar Tabs issues & bugs, most other Tab Managers do not have all these issues, So can finally use TST, without all the ongoing headaches.
My online Reviews of Tree style Tabs Rated ( 2.6 out of 5 Stars).

Best extension ever hands down.

NOTE THIS EDIT: I just noticed configuring TST and following it's advice of changing configuratin 'browser.tabs.multiselect' to true, but it didn't exist! This is 77.0.1 64-bit Linux version and that option didn't exists - I added it, but as it was not there to begin with, I doubt it will be of any help that I added it... Advice?
-- EDIT END --

Awesome! I used to love an old legacy add-on called Tab Mix Plus, which really is an awesome add-on for the tab bar, but now I love the tab tree much more - it's also specifically nice with small netbooks, like my Acer Aspire One D270 (old, I know, but still works marvels if not put under too heavy strain), that have a widescreen display and very low vertical resolution - you can hide the sidebar if you need to, although most of the time it's not a problem, and you save the height of the tab bar in vertical resolution available to show the actual page. With such small displays it really does matter a lot.

Edit: Many people are complaining about the lack of search function. I'm not sure what exactly they mean, I'm completely satisfied with being able to search tabs using the location bar - you can enable this from FireFox settings. However maybe a search function that only shows the matching tabs (and the tab-tree they are part of) in the sidebar - when it's empty it would show them all. I can't think of another way they mean, but I'm really fully content with searching for the tab using location bar anyway :)

give an option to save those whole taps list in a group.so that one can reopen those tabs when ever neccessery

Billant just very helpful.

Decent customization, the large lacking feature is the ability to hide all of your tabs up top.

The main reason I still haven't switched to Chrome

cool addon

This should be the default way of managing tabs for any desktop web browser. A must-have plugin.

Works well enough, but breaks in Nightly 74