Compared to other vertical-tabs extensions, this one has two major advantages: 1) the "tree-tabs" capability can be deactivated (no indentation); 2) hides in full screen. No other extension has both capabilities.
However, it has one annoying bug; sometimes the vertically stacked tabs suddenly become horizontal - just as when they are on top, but still stay within the "sidebar", so they become unusable. Entering the TK2 Options and setting tabs to any other position makes them right again (I can then return them to the original position and they stay fine). I've observed this same problem on two completely different OS-es (Windows and FreeBSD), and on several different computers.
Other then that, the tabs look a bit graphically awkward; that is, they are the original FF rounded tabs placed upon the colored rectangles, placed upon the (dark) background. But that's a minor issue, please fix the problem with tab stacking if possible and I'll raise the rating!
EDIT: It seems the same problem is already reported here: https://github.com/tabkit/tabkit2/issues/104.