Waterfox has a much smaller market share and much smaller budget, and was able to clear this with search partners just by promising not to block ads on them by default.

my point is not actually about search providers, but more generally websites intentionally breaking support for gecko based browsers. waterfox itself is too little, most developers don’t even know about it I think. but firefox is the flagship/reference gecko browser, with more of a measurable number of users. if they implement a good ad blocker in the base browser, that could discourage advertising related sites from serving/supporting this browser.

brave is different in that it uses chromium, which the sites just happen to support already because of chrome. but firefox support is often not a priority even today