I hope this is a good place for my idea. It would be great if Mozilla Team could add two-level tab stacks like in vivaldi. This is a great feature especially for Firefox users who have hundreds of tabs and would like to group them (work, music, projects). I've been using Firefox for years and I like how it's easy to have a lot of tabs and they don't shrink like in e.g. google chrome (terrible!).
I really miss this feature and I hope Mozilla Team will consider this idea and add it to the new version of Firefox.