Do you apply the same rigorous standards to all your purchases?
Do you refuse to buy a drink or a pair of shoes or a travel ticket unless you can ensure that "not a single cent goes to execs" who get "absurdly high, grossly underserved salaries while they butcher the company all the way down"?
Or do you think the Mozilla execs are _uniquely_ greedy to a degree not comparable to that of the execs of Nestlé, Nike, etc.?
(EDIT: This is assuming that you actively want to purchase MDN Plus. If you don't care about the perks but would purchase it solely as a donation, then it's understandable if you give a higher scrutiny to charities than to sellers.)