To me, what they shipped seemed lacking in features to both, with no real improvements.

For example, I sometimes run with hundreds of tabs and my wife has many thousands, at all times. My needs and hers are very different from typical users who have single digits numbers of tabs open, heavily biased toward the low end.

Of course I would prefer TST or Sideberry, but I'm not like most users. For most users, the Firefox experience is superior to Sideberry for its ease of use and fewer failure modes.