Firefox is essentially a dead fish in the sea. Depending on source, Firefox sits at 3-8 % market share. They are much more likely to lose funding because of irrelevancy than some users using AdBlock.
3-8% of the Internet users is 400 million users. I wouldn't call that "dead in the sea". Even 1% of the Internet users is a very significant amount of users. Why obsess with market share? At some point all browsers raised from 0% market share. Choose whatever fits your needs.
Market share is important because if it gets too low, no one tests on or supports your browser. And then coders start only supporting specific features of the monopoly browser and voila, IE6 redux.
