The "customizability" argument took a huge hit years ago when Firefox broke a vast number of extensions not long after promising they would not do so.
These days I use Waterfox with a collection of a dozen or so extensions, update it as infrequently as possible, and pray when I do. I also have Vivaldi for those sites that have issues with Waterfox, and out of habit I also use it for some sites that used to have issues with Waterfox.
The problem with Firefox is the same as the problem with Chrome that is leading people to move to Firefox: browsers keep breaking things. I find it sadly ironic how everyone talks about the web being this platform with unshakable backward compatibility, when in fact from a user perspective it's become quite precarious. If you're a site author and you put something up in an HTML file they'll bend over backwards to support it until kingdom come, but if you're a person using a program on your own computer, be ready to have your workflow come crashing down at any moment when Google or Mozilla decides it's time for a refactor.
With that rant aside, I will say I've overall been quite happy with Waterfox for many years now. The key feature for me in a browser is the ability to support thousands of open tabs. (I know I'm insane, but I gotta be me.) This is achievable in Waterfox with Tab Mix Plus and Auto Tab Discard. Another ace feature is built into (Fire|Water)fox: the box you can uncheck for "let pages override my fonts". This way I see every page in my chosen font. The nearest alternative in Chromium-based browsers is Stylus, which is far less effective.
Overall for me the Waterfox experience is much better than Vivaldi. I just have to cross my fingers and cast myself on the mercy of the cosmos when it comes time to upgrade.