I have to agree with BarberaHudson. Of course market share is a major factor in how much advertising is worth. Companies also look at growth. When Firefox was expanding with no end in sight then signing an expensive deal would have been worth it for Google: if Firefox became de facto then they had an exclusive deal and captured all the eyeballs, and if their immature project took off and became de facto then... they still had all the eyeballs. Once growth starts tailing off the value of marketing drops.
However the funding had in part to do with the marketing aspect, and secondly to buy their way into creating a level playing field. They were perfectly aware of the Microsoft embrace and extend, and they way they tried to lock business users into IE using ActiveX plugins. A completely open source browser dedicated to open standards getting a majority share of the market would ensure that the market remained wide open and ripe for picking once they had advanced their own browser far enough to complete.
The deal probably will be renegotiated but will it be a lot of pain? Possibly not. If the investment was wisely used, refactoring, quashing most of the memory leaks, boosting the JS engine, etc, then if they "broke the back" of getting a great stable core then perhaps a renegotiated deal will be fine for both parties. Google certainly can't afford for Firefox to go away yet. With Google facing ant-trust cases in various countries, helping "promote choice" is not a bad PR move either. As well as providing yet another alternative to Safari on Apple.
PS - AC fuck off. You are wrong and BarberaHudson is right