I see one risk for Firefox: if they block all trackers, e.g. Google Analytics, then it would appear to people viewing browser statistics in Google Analytics that no-one is using Firefox, and thus the developers will be told they don't need to test anything on Firefox.
Or just actively punishing Firefox because blocking tracking is also blocking ads, which is their revenue model. Why spend money optimizing for a browser that doesn't generate revenue? If Firefox becomes the bastion of privacy sensitive people it will become more and more like Tor users, all tainted with the same labels. I mean it's already the case that recaptcha will more likely trigger on Firefox than Chrome, aski…
It's not, the Internet is the only medium where it's assumed that tracking/targeting is necessary for advertising. TV, Magazines, radio, podcasts, cinema don't track users.