Okay, I deleted my previous review. It really didn't work. My guess is because that time I had my session tabs messed up by Tab Manager extension, which I used to organize tabs between windows and it seems has adjusted moved tab properties to that none of session managers worked.
Now I re-tested again and basically it works.
HOWEVER,
Sessionic is indeed buggy:
1) It kills CPU and Options page takes about 5 minutes to load.
(I have 20 windows open, 10 regular and 10 private, 20k tabs open - all discarded by Auto Tab Discard, 0 pinned tabs)
I wonder why opening menu could cause such a lag with Sessionic extension?
Options menu should show just Options, but not do any unexpected heavy computation.

2) In Firefox 100% page view Options page GUI is visible only upper half. I have to adjust zoom to 50% to view full GUI and then text is small.
(1920x1080, Screen Layout & Scale -> Scale is set to 150%)

3) Would be great to have auto export to file function, because unexpected browser crash can potentially break extension data.

If these issues get resolved then this extension could beat Tab Session Manager.

Update:
Didn't confirm many opened tabs cause issue with Sessionic, because for me Options still freezes for several minutes when no tabs opened. But in FF taskmanager I see extensions eating 100% CPU. If I kill extensions process, FF asks me to reload and restore crashed Sessionic Options tab, then it loads normal and fast.
I tried creating new profile and copied all cached data, except extensions, but it worked fine.
So my guess is it may be related to extensions installed.
ah, I forgot I had to enable extended filters in uBO and noscript. I will try again and see if it reproduces problem.