I don't follow this line of reasoning. Google pays Mozilla for the search traffic. If Firefox overtook Chrome in market share, Mozilla's position would become even _more_ favourable and they could command a larger sum from Google. If Google threatened to end the agreement, Mozilla could simply walk to Bing/DuckDuckGo or whoever else.
Right now, Google pays more than Bing or DuckDuckGo.
Perhaps Google just has mountains of spare cash, which DDG doesn't. Perhaps Google gets extra value as FF both provides search traffic, and keeps competition regulators off their back. Perhaps Bing thinks if FF changed the default search engine away from Google, 95% users would change it right back.
But if Bing is only willing to pay 70% of what Google pays - could Mozilla survive losing that much income? Or would it trigger a death spiral, with less money meaning less development meaning lower market share?
