Firefox isn't really making enough in donations to maintain a major piece of web infrastructure. So it gets search revenue from Google, which people complain about (and may not be long term sustainable). So they go make other paid for things looking for revenue, which people complain about.
If the user donations aren’t enough, surely their main revenue source—the 100s of millions of dollars they get from Google—is? Or do they need billions just to maintain a competent browser?