The big question is:
Does the money go to Firefox or to funny projects and (what I consider) insane exec salaries?
So yeah, find me a way to finance Firefox only, and not the funny projects, and to ensure that not a single cent goes to their execs (neither directly nor through some creative accounting, where they reduce engineer salaries to offset the cashflow from this new stream, and pocket the savings), and you have my 10 bucks a month for the next 8 years (and possibly longer, but let's see what they do in 8 years). I won't move the goalposts and I'll make good on my promise.
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.)
I would bet you that most people are in the latter group, not the former. I certainly almost never purchase such services from ordinary companies, as I don't see sufficient value in them.