True, but what happens when Firefox's marketshare decreases to the point where the amount of traffic lost by not having the Google deal stops mattering to Google?

If Google does the math one day, and determines that they won't lose out anymore by not paying Firefox they'll stop paying.

It's revenue share based, so the cost to google is the time it takes to renew the deal. This is a fixed cost that doesn't depend on the market share of Firefox.