Even though non-profit organizations like Mozilla do not seek to maximize financial gain (by definition), they often seek to maximize their relevance in the world. As a result, they ARE susceptible to corruption: most if not all are willing to "compromise" -- that is, sacrifice their mission and values -- in order to remain "important" in the eyes of society.
The folks running Mozilla are sacrificing the organization's mission and values because they're afraid of losing market share. They do not want Firefox to become a niche platform.