I've been using TST for about 6 weeks, and it's worked fairly well, but it's got at least two annoying problems.
First, when I open a link on a page and specify that it's to be put into a new tab, TST will occasionally create the new tab in some random location farther down the tab stack (but not as a child of the active tab). Trying to move that new tab back under the active tab is fruitless--it always jumps back to the random location.
Second, whenever I restart FF, TST doesn't retain the tree structure that existed before the restart. The first group is always as it should be, but beginning with the second group, all subsequent groups become child branches of the second group or some other subsequent group. It's a real mess, and it takes me a good 3-4 minutes to manually put things back the way they should be.
Hopefully, all this will go away in time. I should note that I'm running Tab Session Manager, though I don't think the problems started when I installed that (about 4 weeks ago).