I will quote a comment on HN from a Mozilla employee:

You shouldn’t donate. The donations don’t go to MoCo which is the for profit subsidiary of Mozilla Foundation under which FF is developed. All the donations go to MoFo, where they barely covered Mitchell’s salary and what was left went to her pet political projects. People are pretty unaware/misled about how Mozilla is structured/funded. The only way you can contribute to FF directly is to keep and use Google as the default search engine.