Very good at saving tab states, even in disastrous scenarios. However, it does not play well with "Tiled Tab Groups". While Firefox's default session save function (in the rare scenarios in which it actually functions properly) will restore with the tab groups intact, Tab Session Manager will destroy the group states, and loads them all in a single group. I'd suggest then that it simply doesn't seem to respect hidden tab statuses? That said, still the best we've got in Quantum by a mile so far.