Any browser is going to struggle against the top two behemoths, so I'm not sure why exactly that would be an argument to just accept it and continue using products by companies that have lost your trust. Are end users supposed to compromise their ethics just because web developers aren't in the habit of testing against their browser of choice yet? That just seems like an awfully 'silicon-valley-centric' view of the world.
What does this "success" thing have to do with ditching a product that had violated your trust anyway? Are businesses in most other industries viewed this way? Am I expected to stop buying food from local producers just because the big-box food suppliers are more ubiquitous?
To me, all that matters is sustainability, and IceCat has been going strong for 12 years, PaleMoon has been consistently developed for 8 years, and Waterfox for 6 years. So that's enough for me to at least give them a shot.