In Firefox you still have the tabs at the top after pressing F11. That's not true fullscreen. I can understand why users who wish to modify the code of their browser object to proprietary code of Vivaldi. For the ordinary user, though, interested in performance and not politics, it is great. It's not my default browser (I use Firefox, e.g., to access F95) but it is my gaming browser, hands down.
I'm sorry, but that's just wrong. You can access the tabs by moving your cursor to the top of the screen, but as soon as focus is released, the tabs are hidden, and you do get true fullscreen. You must have a weird tab or titlebar setting enabled if they don't auto-hide. Also, you can customize the location of the button for fullscreen too, along with zoom in/out, just like other browsers.
Also, supporting open source is indeed political, but it's not only about modifying code yourself, which I barely ever do either. I just use the builds Mozilla or 3rd party devs release. You're right; we don't need to get into reasoning behind it here. In the end, it's personal preference. Like I said, if you like Vivaldi and its features, then go for it; I just couldn't help but refute what I considered bad info.