Dillo, NetSurf.
They are fine if you mostly use sites like HN and other mostly-static content, maybe with some forms interaction. Those are HTML4-ish "document web" browsers. There's also all the text-based ones that those who want to send an even stronger message about how much they hate the "modern web" can use.
That leaves Edge, Chrome, Opera and Vivaldi
Because you even mentioned them, I don't think you understand the question. Let me rephrase it: who will develop, and have a sustainable business model for a new browser engine and break Chromium's monoculture, other than Firefox?
I see no proof of this.
>> is struggling
Money-wise, no. Management-wise, hell-to-the-yes.
Another downside to Vivaldi is they don’t push out many releases meaning that their security is likely lagging behind. My assumption on this is their custom UI takes a while to build after a new release.
Edge and Chrome both push out timely security updates.
EDIT: Typo.
After looking for weeks I created a list of categories... And it depends on what are you willing to sacrifice:
Just want a plain browser, without logins, synchronization, without ANY external connections etc:
Ungoogled-chromium (Windows, Linux...) and Bromite (Android).
Don't care about full web compatibility or speed but keep your privacy under control:
Waterfox Classic and Palemoon (Windows and Linux).
Don't care about Open Source vs Closed Source licensing (but still preserve some privacy):
Vivaldi (the most customizable Chromium-based browser out there but the UI even after several improvements feels a bit slower than other browsers).
Don't care about privacy... Then it's quite easy:
Edge, Chrome, Opera.
If you are into cryptocurrencies or need a nice browser with decent adblocking on iOS: Brave.
I decided to use Vivaldi since I like to personalize the looks of my browser quite a lot.
The best recommendation is to try everything for a while and settle with the one you feel most comfortable.