Vivaldi doesn't even have a fraction of the customizability features I require. I tried to get it into a configuration that would work for me, but it wasn't possible from within the UI itself, and I wasn't interested enough to find out if there are more things that can be done outside of the UI. It may have more options than any other browser out of the box, but I don't use any of them in that configuration anyway.
Nothing comes close to Firefox with a full complement of extensions as far as customizability... the one and only thing Firefox still does better than anyone else. It's not faster, smoother, more secure, or better on memory (as far as I know). It has the most powerful addons, and the largest library of them of any browser.
So, naturally, killing that off and adopting Chrome extensions is next on the Firefox suicide plan.
Back when I was using Mozilla Suite, when a handful of Mozilla devs decided to fork the browser component from within the Suite and make a standalone, lean and mean browser, the goal was to fight the corporate giant trying to dominate the web. Back then it was Microsoft. The devs never worried much about the options that IE had or how its UI looked-- they just made Mozilla/Phoenix/Firebird/Firefox the best browser they knew how to make. It didn't have to defeat MS in market share (as they are saying now, apparently)... the goal was simply to provide an alternative that was good enough to keep Microsoft from owning everything.
It took some time, but if you look at the charts of browser market share from 2000 to present, you will see that FF began to make some serious inroads into IE's dominance. IE market share was declining and Firefox was on the way up when Chrome arrived. It stole much of Firefox's thunder; so began the decline.
If only we could get back THAT Mozilla. They didn't conclude that since IE had 95% market share, it meant that people really, really liked IE, and that the way to gain market share was to copy IE in every way and eliminate any superior features that would serve to distinguish it from its corporate competitor. In doing so, the devs would have thought they were making it as easy as possible for people who were completely satisfied with IE and whose needs were being perfectly met by IE to switch to Firefox.
The tabbed browsing would have had to go, of course. IE didn't have it, and since everyone uses IE, that must mean that's the correct way for it to be. If the FF market share doesn't rise, the problem must be that it's still not a good enough copy of IE.
Firefox never would have began to erode IE's dominance and forced THEM to introduce tabs if they'd had that insane mindset back then. Obsessing about market share and the psychology of the users who are already used to IE and who expect things to be a certain way would have put the cart before the horse. Sure, you can build a copy of IE, and if people who really and truly liked IE DID decide to switch, it would be relatively painless. One question: If they really and truly like IE, what is going to convince them to switch to your IE copy when they are already using the genuine article? One more question: If your goal is to prevent the corporate dominance of the web, have you really accomplished that if your product follows the corporate design of IE as much as the actual IE?
Mozilla lost the plot years ago, and they're doubling down on stupid every chance they get.