Not only a bigger market share. What's keeping Firefox alive is the financial support they get from Google. If enough people move from Chrome to Firefox without Firefox also securing finances from elsewhere, Google could easily kill Firefox by just not giving them money and we'd all be left with just Chromium.
I think the real reason Google is funding Firefox is because they're afraid of being targeted in antitrust lawsuits. As long as Firefox is around, they have someone they can point to, to say they're not a monopoly.