I just installed it yesterday and so far it f*ck*d everything up it could.

1. I moved some tabs into a new window to convert it into a tab-group but I didn't find any option that offered specifically that. So I choose "Create new group" in the hopes it would do that OR open a new window where I could move said tabs afterwards. It emptied out the current window without asking for confirmation or even a warning instead. :-(

2. Needed to restart my PC so I "closed all [Firefox] windows on this screen" (or however this option is called in English Windows). I expected Firefox to restore both windows after I started it again. Instead it just opened the new little group from 1. and an empty group. My (ungrouped?) window with ~200 tabs was nowhere to be found. :-(

Luckily it seems I was able to restore most tabs that had vanished in both cases.