Mozilla gets the majority of their revenue from Google paying them to keep their search engine as the default. I'm OK with this: the search engine is visible every time you use it, so it's impossible to "forget" to change it if you care about privacy, and the setting is relatively easy to find and change. Also, without this funding, Firefox would likely not exist. It's the price we pay for a good, free browser.
Of course, many have postulated that Google's motivation to pay Mozilla all that money is actually to ensure that Mozilla remains in business, rather than to directly increase Google's own revenue with the added search traffic. By keeping Mozilla in business, Google can more convincingly argue that they do not have a monopoly on browsers. It may be the case that without this argument, Chrome could be subject to regulations that could benefit consumer privacy, but after watching all the recent congressional hearings involving big tech companies, I have very little faith in that happening any time soon. It's better to keep Mozilla around for now, and the only way to do that without turning Mozilla into another Google is for Google to essentially subsidize them.