IMO ~+10%pt just provide Google with a thicker armor against antitrust lawsuits. "Hey hey hey, can't sue us! We have a competitor with ~15% of the market! And we helped them get there! Look at the 500 million we give them per year!".
If Mozilla wanted to be a threat to Google, IMO they could, but they'd rather pay their CEO 5M, fire a few hundred engineers, and spend a fraction of their Google money on Firefox.