https://github.com/mbnuqw/sidebery https://addons.mozilla.org/en-US/firefox/addon/sidebery/
It's still weirdly slow, it somehow takes thousands of milliseconds (entire seconds!) to display the tab list on startup, but much less buggy than TST.
The styles editor is very nice. Changing the fonts to -apple-system made it blend in well with the OS.
Some things are in unexpected places (for example I expected to reopen a closed tab with right click outside an open tab - reopen closed tab, but it's in the trash icon at the bottom), but I think they are in better places. Sidebery is better designed.
Sidebery can also save your tabs automatically, something I used another extension for (Tab Extension Manager, which loads very slowly)
I submitted a feature request a year ago but got no traction, and now I just expand and re-close groups before mass closing to double check if I need anything.