Background: I used Opera Presto as my main browser from 2007-2014, and Vivaldi since 2016 (2015 being a mix of Opera, Firefox, and Vivaldi). I've used Firefox as my main browser at work for most of the past four years.
One big factor for me is the customizability without having to rely on extensions. I trust Vivaldi (and von Tetzchner), and don't have to worry about whether I trust extension makers. The customization options in Vivaldi tend to work pretty darn well too, which can be more hit-and-miss with third party ones in Firefox.
Single-key shortcuts are also a big win for Vivaldi for me. I got used to these in Opera and they're great for efficiency while web browsing. Maybe I can get them with an extension for Firefox, but it's enabling one check box in Vivaldi. It's similar to why some people like vim plugins for browsers, but with less vim expertise required.
I've also seen Firefox remove a lot of features over the years. XUL extensions being the big one, but there are others. Firefox Hello. Firefox Panorama, just as I was starting to take a liking to it. Vivaldi has a philosophy of adding features that let you customize the browser to how you like it, and if only 0.2% of users use that feature but it makes them really happy, that's fine. Firefox will remove features if not enough users adopt it. Thus my trust that Firefox is going to keep features that I like has been lowered over the years, making me more hesitant to try new Firefox features, and generally more hesitant about Firefox.
Although I'm considering switching part of my home browsing back to Firefox because of its ability to not autoplay videos. Too many sites have those nowadays (and too often, self-hosted and not as third-party ads that are easily blocked), and Blink-based browsers can't stop them, and I'm getting pretty tired of having to pause/hide videos all the time so I can focus on reading articles.
