In this case, with Mozilla Corporation as well as the budget mainly coming from google instead of user donations, you have a point. They have a financial interest in higher user numbers. Your statement is however not universally true. If you have a large enough userbase to finance development, or are an open source project, as a non-profit growth is not something you need to spend money on. That really only matters if you have a profit interest. Differently put, you dont need everyone to use your software, it is enough that its available for people who need it. With more growth you get a litany of new problem with having to do something with the incoming money as a non profit. That often means feature bloat, while, "keep it simple stupid" would have led to a better solution. Not to mention adding features the new userbase requests. In the worst case you end up with a completely different product which for many of the old userbase and volunteers is just as bad as if the project was canceled altogether.

All of this not to mention that, while it makes sense for Firefox, the thought that my donation to develop Firefox ended up in some marketing budget irks me deep down.