They make a large amount from Google paying them to be the default search engine. Also they have been making additional projects that can be subscribed to as add-ons for Firefox (like a VPN and an email forwarding service that allows you to make fake email addresses or phone numbers to use on sites that will forward the messages to your real inbox/phone). You can use a limited version of the email thing without paying though so it is easy to try out. And they are always ready to take donations of any size and can be reoccurring. I personally pay .99/month for the email service even though I don't use it often. As it is nice to have if I need it, and it is basically a donation at that point. lol.

Here are links to those products if you care to read more about them or at least see pricing.
https://www.mozilla.org/en-US/products/vpn/

https://relay.firefox.com/

But even just making a point to donate some one-offs here and there does help in small ways to keep a real option in browsers that isn't just another Chromium-based project.
https://donate.mozilla.org/en-US/

Everyone hated when IE was the only browser that sites were coded for, and we are seeing more and more Chromium only sites. Which means a bad vulnerability in Chromium will impact all the browsers based on it. Also privacy add-ons for Firefox tend to work better and block ads well.

Mozilla seems to be pretty transparent. You can see their financial statement at State of Mozilla.

1. Donations

2. Getting payed by google to make it their default search engine.

They get paid by Google to feature their search engine as the default primary search engine. In Fennec, the non-google-play version of mobile browser Firefox, Duckduckgo is set as default, even though both versions are maintained by Mozilla, the non-profit organization behind Firefox.

I donate every. single. month.

Mainly a non profit