but if they are already taking money from Google, why should I donate to Firefox?
IMO companies that aren't getting direct money from ad businesses deserve my donations more.
There's no guarantee that reaching independence from Google will stop Firefox from getting Google money, disabling features that made Firefox different or cutting jobs.
Most of their revenue comes from Google (for providing it as the default search engine in the browser) because they can't get enough revenue from donations to pay for their work. If they got enough money from donations, they wouldn't need to rely on Google's spare change.
Do you realize that 0 donations go to Firefox development?
Firefox development is done by Mozilla Corporation (that doesn't accept any donations, AFAIK). A part of their earnings are given to their owner, Mozilla Foundation, that you are suggested to donate to, and it may not be a bad idea, but it does not finance Firefox development in any way.
IMO companies that aren't getting direct money from ad businesses deserve my donations more.
There's no guarantee that reaching independence from Google will stop Firefox from getting Google money, disabling features that made Firefox different or cutting jobs.