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.

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

after using this, and accidentally found another add-ons called Sidebery. I think this needs to be developed further, because what is here is in the sidebery and even more features. Don't know why Firefox recommends these add-ons more than Sidebery

UPDATE 3-STARS: Frustrating that a core function to group selected tabs has been removed and now requires a separate extension to be installed.

ORIGINAL 5-STARS: Great extension, love that it can be extended by other extensions too.

Jul 1, 2021 - MANY BUGS:
- frequently crashes and "restarts" add-on and sidebar... I tend to have 500+ tabs open (currently 1600+)

- for some reason sometimes tab tree opens into a new window when I am simply clicking tabs in sidebar.... and I DO NOT even have the "Tree Style Tab in Separate Window" option added

Enhancements needed:
- need to improve the highlighting of which tab is the current active tab.... something like a black border... including when using with Colored Tabs

- need a command to be able to separate 1st tab from tree... instead you have to move all the other sub-tabs

Nov 28, 2019 - please add search function/filter to tabs - I see this was just added as of Jun 2021 - YAY! (looks like still beta version though but nice move!)

Aug 9, 2019 - Please add keyboard shortcut for Mac, preferably one-handed, e.g. OPT-1

Seems when i restart firefox the connection to the parent tab is dropped, so structure end up flat again. A bit annoying, and looses a big part of its function. Otherwise a nice addon with potential.

It's wonderful to work with tabs in a tree in the sidebar. However, there's a frequent bug appearing: sometimes the last tab or the one before the last one bring you to the incorrect tab (and reloading the extension/sidebar doesn't work). Other times it won't let you change tabs at all (clicking, ctrl+tab works fine), or hang unexpectedly until you reload it. It's not reliable, but when it works is quite useful.

I have three recurring bugs, and they're ones reported by other users. Firstly, the tab sidebar often doesn't load and just shows the loading indicator forever. Secondly, it can get into a mood where clicking one tab switches to a completely different tab (often the tab directly before it in the tab order) (clicking the tab in Firefox's default top tab bar instead works as expected, so the issue is definitely with this add-on). Thirdly, it often forgets and rearranges child tabs with no apparent logic, changing the order and hierarchy completely.

I rely heavily on this add-on so I'm hoping they can fix these issues. It would be an easy five stars if I could just trust it without worrying whether or not it'll even load! I want to remove the default tab bar and just use this add-on, but at present I can't trust the add-on to work properly.

Not works very well on mac firefox, no any tabs show up but just a loading indicator keeps loading forever. This doesn't happens everytime, but still very high frequency. Althrough I can get rid of this by disable and re-enable it, it's still anoying

WARNING: This addons is currently very buggy. Usually, it's a simple case of clicking a tab and a completely different tab is selected, every now and then. This is solved by going into Tree Style Tab's settings and unchecking and checking the "optimise tree restoration with cache" option. But today, I leave Tree Style Tab at the wayside, as my entire 70+ tab tree structure was completely demolished when I opened up Firefox for the day, only to see all tabs being considered direct children of 1 other tab, disregarding any previous tree structures. Disregarding the fact that I should probably get my tabs sorted out, it is clear that Tree Style Tab, currently at version 2.8.5, is too flawed for my uses. Tread with caution.

Can you
1) create an option to FULLY disable tree structure?
2) Add folders?
I just want to have vertical tabs, no any trees.

You need to urgently remove/move/add possibility to remove the "Close tab options" menu in the context menu. Now it can be accidentally triggered to close all tabs when actually trying to reopen closed tab! Just lost over 50 tabs doing so!

Otherwise the add-on is great and recommended for anyone with high number of tabs open.

It'd be a true gem, if someone could change the tabnames so that you don't see the

Nuts...doesn't work with Colorful Tabs like it did in the old Firefox.

В Tab Tree было авто скрытие панели владок, переключение по клику между двумя вкладками. Этого всего очень не хватает.

This is a love-hate relationship. The concept is great. The implementation is horrible.
Sometimes this add-on just mixes up the hierarchy of my tabs and I painfully have to reorder them by hand. This can be a lot of a work since this add-on espeacially makes sense when you use many tabs.