Mozilla made 530M in 2016 - they're a company. Firefox is is a strategical business decision. Their interest in the web is the exact same interest as Google's in the web. $$$.
Mozilla is constrained by its non-profit structure, its open source licensing, and its high organizational transparency. While Mozilla's interests can be analyzed in terms of $$$ (or "power", or ...), such analysis is reductive and lossy. It's not meaningless, but it offers limited insight.