It is a little bit buggy. Sometimes tabs will open multiple times and make huge mess.

Put all my tabs into a group, restarted firefox, the group no longer exists.

STAY AWAY IF YOU WANT TO KEEP YOUR TABS!!!

works well, feels like using safari tab groups sidebar

Simple Tab Groups? I suppose it's simple to use but it's anything but simple in functionality! Wow. What can I say? Firefox actually works, unlike Safari and this extension actually works, unlike Safari's tab groups. This will change how I work and browse. Thanks!

I did not install this extension because once I saw how much data it collected and what it wanted access to, I was quick not to allow it. Why does it need access to recently closed tabs, access to download files, read all website data? C'mon... this was one of the only tab grouping extensions I would have use too...

too intrusive. OneTab does not requires so many permissions.
https://addons.mozilla.org/en-US/firefox/addon/onetab/

It would be great if the developer could add "subgroup" functionality to the extension. I right now have 65 groups open lol. I would love to be able to put each groups into categories. For example., if I am doing research on topic X, Y, and Z, and have groups named X,Y, and Z (respectively), I would love to be able to put all three groups under one group called "research". Hope that makes sense. I'd hazard a guess that this shouldn't be too difficult to implement. Many thanks to the developer for this extension, it has made research so much easier.

My comments in Portuguese and English:

Justamente por ser "Simple" Tab Groups, é prático para usuários comuns que precisam de praticidade para organizar numerosas abas abertas. Você cria grupos e os identifica como achar melhor, e em seguida arrasta para dentro de cada grupo as abas abertas, de acordo com os assuntos.

Por outro lado, uma grande desvantagem desse complemento é que ele força você a ocultar as guias agrupadas.
No agrupamento de guias do Chrome, seu grupo aparece como uma única guia que pode ser facilmente expandida e recolhida quando você deseja mostrar/ocultar todas as guias pertencentes a este grupo.

Precisely because it is "Simple", it's practical for common users who needs practicality to organize numerous open tabs. You create groups and identify them how to find better, and then drag into each group the open tabs according to the subjects.

However, a major drawback of this addon is that it forces you to hide the tabs that are grouped. In Chrome's guide grouping, your group appears as a single guide that can be easily expanded and collected when you want to show/hide all guides belonging to this group.

Private mode is NOT SUPPORTED. What a bummer!