For better or worse, it just "looks bad" when a non-profit executive makes a seven figure salary. And when people ask "could we set up an ombuds/ethics committee/advisory board to prevent Mozilla from doing this?" The answer is the same: no, that wouldn't really work.
The donations are for the foundation, the corporation makes a lot of money off their search engine deal. People often bring these things up as they sound bad, so if you're going to to repeat it add the necessary context and avoid the simple misunderstandings.
>could we set up an ombuds/ethics committee/advisory board to prevent Mozilla from doing this
They have a board that picks the corporation's CEO and sets their salary. I doubt you want the monetary sources to control this board, as that would give Google almost total control.
That $2.4 million dollar salary is from 2018, a year when Mozilla made most of it's money from Yahoo. This is relevant as that Yahoo deal saw Mozilla making between 100-250 million dollars more a year than their deals with Google. That is what resulted in the board increasing their pay, and after the following Google deal was far lower many of their executives were forced out.