This is the main reason I stay away from Vivaldi; using Firefox is, for all of Mozilla's borderline comical mismanagement, a protest vote against Blink (and previously, Chromium).
Vivaldi is almost certainly the best Blink browser, and I'd certainly use it if only Blink browsers were viable. As long as that's not the case, I am, like you, using something based on Firefox; in my case, Zen.
This is why I use Zen. All the benefits of Vivaldi, with the peace of mind supporting a Mozilla stack.
This has been a lost cause for the past decade or so. Web developers don't target Firefox anymore because a 5% share isn't enough to matter.
Both projects (Chromium and Firefox) are open, so it's like Linux vs FreeBSD, but at least FreeBSD has a clear licensing advantage.
Firefox is controlled opposition practically owned by Google. Follow the money.
Ladybird seems to be the only hope, once available.