This is the features page for Vivaldi's tabs. And it has two or three subpages that explain some features in more detail:

https://vivaldi.com/blog/how-to/tab-management-is-our-love-language/

It also offers the sidebar on top of all the other features (although the FF plugins are a bit more powerful in the sidebars). It also lets you hide the tab bar completely, unlike FF where the sidepanel exists in addition to the the always visible tab bar... that's the definition of redundancy.

As you say, to each his own. Unfortunately, I am just that little bit less productive when using FF.

Edit: minor edits for clarity