You gave the example of Firefox. In fact, the Mozilla organization, which manages Firefox has plenty of paid employees. See https://careers.mozilla.org/ https:/

You gave the example of Firefox. In fact, the Mozilla organization, which manages Firefox has plenty of paid employees. See https://careers.mozilla.org/ https://careers.mozilla.org/. The same is true for some other major open-source projects such as the Linux Foundation or LetsEncrypt. Being an employee at one of these organizations means you are literally being paid to contribute to open source projects.