The sentiment below is correct, there should not be a real concern here for Mozilla. Here's how you should think about it from a business perspective. Most (but not all) companies should be concerned about their bargaining power with customers when revenue is highly concentrated. This concern comes from the fact that customers can threaten to leave, which will significantly decrease revenue and be difficult to replace. Consequently, many companies will lower prices under the logic that retaining 90% of that revenue is better than the results from trying to replace it with smaller revenue opportunities.
However, this is far from the case for Mozilla. Google pays Mozilla in order to be the DEFAULT search engine (which is valuable on the theory of consumer inertia). The default position is a limited commodity; there is only one default position and, therefore, by doing business with Google they are passing on other business. If Google left then Mozilla would be able to sell that default position to someone else (probably Yahoo or Microsoft) and would be able to replace most of that revenue with comparable revenue very quickly.
I would argue Google is in the more difficult position here. Google has a some (not a lot) concentration of their acquired traffic from Mozilla (I think ~10%), and that is not a limited commodity. If there were other comparable traffic acquisition opportunities available, Google would pursue them. Google is not necessarily passing on other business because of their business with Mozilla (while, as explained above, Mozilla is passing on other business because of their Google deal). So presumably if Mozilla and Google part ways, that Google business (revenue, profit etc.) would be lost (at least for now) and take some time / effort to replace. While Mozilla’s power over Google is definitely small, I would say with certainty Mozilla should not be concerned about their Google concentration in this scenario.