UPDATE 7/12/21:

Since my last update a year ago, pretty much all problems of which I was aware have been fixed.

But since the last update (or maybe the one before that--but recently, in any case), a new one has cropped up:

When coming back to a Firefox session from Windows Sleep mode, TST has unloaded, and the main toolbar icon is gone. I've also seen this happen after a Firefox restart or crash. Going into the View menu to re-enable TST doesn't work, as it's also gone from there.

I've noticed at least one recent review that reports this problem when "starting Firefox."

The only work-around I've found is to open the Add-ons and Themes page, disable TST, re-enable TST, then go to the View menu to re-load TST in the sidebar.

It's a recoverable problem, but it's a pain to have to do this every day (and some times two or three times a day).

UPDATE 7-21-20:

Since the last Firefox update (to 78.0.2 64-bit), almost every time I left-click on a tab in the sidebar, instead of just selecting the tab, TST executes the "Move tab to new window" action. If the tab has children, the entire tree is moved to a new window. (I say "almost every time" as there may be one time in about ten that TST does not do this).

Needless to say, this is extremely annoying, as I then have to move my tabs (one at a time) from the new window back into the original window.

If I remember to do so, I can select tabs normally from the top tab bar, but muscle memory is ingrained for TST's sidebar, at this point, so I frequently forget to do that.

I don't know if this problem is due to a change in Firefox or in TST, as I'm clear on how their latest updates might be interacting. I can say that TST v3.5.13 did NOT fix the problem.

UPDATE 12-31-19:

The parent/child tab separation happens rarely now (and it's been a few months since I've seen it happen, so that bug may have been squashed.

ORIGINAL REVIEW:

Works pretty well, but sometimes child tabs get separated from the parent tab and other child tabs, with non-associated tabs in between. I've not seen a pattern to when this happens, unfortunately. At least the tabs are lost, but it can get confusing.