Firefox and Mozilla should be scrutinized to some degree. Firefox may be free of charge, and so I agree Mozilla is not directly beholden to its non-paying users. But it is beholden to its mission statement, and it's fair to criticize Mozilla for anything that might violate that statement.
My understanding is that a "donate to Firefox" option is a logistical nightmare, would only be allowed in limited jurisdictions, and is not expected to earn all that much revenue. So in effect, the "donate to Firefox" button would end up mostly supporting the continued existence of the "donate to Firefox" button (lawyers ain't cheap!)
Purchasing services even if you don't use them (eg Firefox VPN; I'm not sure what else has been released yet) is the most direct way of donating to Mozilla Corporation and thereby funding Firefox development. The most important component of which is my personal salary. Thanks in advance!