although to me in some ways Otter, Vivaldi, and Opera/Blink are superior to Firefox
"To me in some ways"
I assume that not only for you but for many other people in many ways.

Don't get me wrong Frenzie. It's not my hobby to argue just for the sake of arguing.
All I want to make clear is that priorities and ways of using a browser can (and mostly does) differ from user to user.
Some handy features a browser might offer can be very important to you while less important to me and vice versa.
Theoretically at least, the more options and features a browser will offer, the better the chances to attract users with different priorities.
Theoretically because theory doesn't apply always to real life. Most used browser is (was?) Internet Explorer which probably will be (allready is?) surpassed by Chrome. The wide usage of these two browsers has little to nothing to do with options and features. Neither with users' priorities but rather with the lack of them.
However Opera Presto is a good example for attracting both of us despite of different priorities.

Speaking of priorities
Some of the vital and very basic features for me and probably of less or no importance to you (correct me please if I'm wrong):
- Access on the fly for toggling cookies
- Access on the fly for toggling scripting
- Access on the fly for toggling proxy
These are just the very basic ones.
AFAIK there are only 3 browsers offering these very basic features. Opera Presto, K-Meleon and Firefox (with the respective extensions).
Among these 3 browsers only 2 can
instantly stop scripts on a page: Opera Presto and K-Meleon.
Among these 3 browsers there is only 1 browser where you can set up different proxies for different tasks and switch between them: K-Meleon.
Since one picture tells more than thousand words:
screenshotThe way I've customised K-Meleons GUI to fit
my needs. (no extensions involved, all buttons are optional you can remove them or add other ones for other tasks, almost no limits for customising the UI)
Granulated options for settings, rendering speed and resource usage might be some features which both of us are caring about.