I know it’s a big issue for many given the bad news and crap Google does, but Mozilla is actively taking away features from Gecko whereas Vivaldi, Microsoft, Google and Samsung are improving chromium. I find it hard to make Firefox behave how I want it to when browsers like Vivaldi or even Edge make it easy.
I know a common sentiment I hear (at least on Reddit) is “Why didn’t so and so’s browser use Gecko?” but when a company or person does, a lot of /r/Firefox and /r/Linux users will get upset that they are forking Firefox instead of adding to Firefox/Gecko.