If you don't plan to be poor, then you need many paying customers.
For a browser, the customers could be either end users, corporations with many end users, or marketers who want to sell to the end users.
The latter case is Chrome, which works through a long chain of Google services.
The corporate case used to be IE, geared towards deep windows integration.
The case of the paying end users used to be the original Netscape. It did not work well for a number of reasons. Paying for a product when free-as-beer alternatives are available takes understanding, which is sorely lacking outside some segments of tech circles.
I personally buy Mozilla a figurative pizza by a yearly donation. I do it because I understand their importance and feel grateful for their products. If more people did that it would help, but most people who are to benefit from Mozilla's efforts are not technical.