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).
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.
This is why I use Zen. All the benefits of Vivaldi, with the peace of mind supporting a Mozilla stack.
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.
Firefox is controlled opposition practically owned by Google. Follow the money.
Ladybird seems to be the only hope, once available.