TBH it's a really good point. Any time Mozilla has tried to appeal to users in a way that would actually be profitable their users have lost their minds over it.
The issue is that donating money to Mozilla does nothing for Firefox; Personally I use Firefox on everything (home, work, phone); Firefox is the browser for my family too - much the same (Firefox on everything, phones included).
If I could/had to pay/donate for it - I'd gladly do, but it's virtually impossible.
AFAIK income from particular Mozilla products isn't earmarked for spending on specifically those products, so buying non-Firefox products can provide income that can be used to fund Firefox development.
That said, I think income from Firefox's default search engine pretty much dwarfs any income that could potentially be gained from donations/buy-to-support.
