It still doesn't nest tabs like Tree-Style tabs addin for FF though right? When they get that I'll switch at least part time, FF for me is getting so laggy (due to inefficient JS on certain websites like twitter) it is extremely painful to use.

EDIT:

https://forum.vivaldi.net/topic/3255/tree-style-tab/16

One page 4:

https://forum.vivaldi.net/topic/3255/tree-style-tab/67

"I do know that internally, there is discussion as to various ways TST might be implemented. Nothing has been settled on, but there is discussion."