I don't think we all agree on what the impact of Firefox market share evaporation is, but my main concern is Google having totalitarian control of the internet: the Browser and Search.
To combat that, I would focus on decreasing the reliance on browser specific tech (specifically: javascript, but also html/css, and maybe HTTP to a certain degree), ensuring that that the web can be browsed without being at the mercy of the client. Return to servers front-loading the entirety of web content. Obviously this would require a massive cultural shift.... The ethics of web dev would have to be shared by vegan-comparable evangelists. (I realize how unrealistic this is after typing it)
I would also try to produce a PoC search engine that mercilessly punishes the ad-driven web, which - we've all noticed- is becoming increasingly pervasive.
Accomplishing these two things, I wouldn't be too concerned about Firefox specifically. Obviously these are immense- perhaps unrealistic goals- but it's what comes to mind.