This is a bit of a silly argument, because money is fungible and dollars are dollars. Donating money to the Foundation means less of the Corporation's dollars have to go to supporting the Foundation (Edited to add: which they do via dividend payments), and so more of the Corporation's dollars go to Firefox.
I don't disagree with your concept in theory, but I also don't think there is any evidence to suggest that MoCo's trademark royalty payments (as IIRC that's how they are structured according the financial docs) are adjusted based on MoFo's needs.
Not explicitly, but I do recall meetings where the announcement was along the lines of "we are choosing to make a dividend payment to fund the Foundation work..." which implies some amount of adjustment based on needs.
I do donate to the foundation, but as others have said, that's a separate organization. I guess I could "donate" to Firefox by getting a VPN subscription, but I don't want a VPN and I think that would send the wrong message. I would like to monetarily indicate that I value Firefox specifically.
Maybe you should contribute to one of the many "fix bugs for money" websites where you can specifically donate money to get bugs in open source products fixed. It's even better because it doesn't have to be a Mozilla employee who does the actual fix.
That's very very cool, but I too would specifically like to pay Mozilla for Firefox. I have been a dedicated user since Phoenix 0.2 and it is near to my heart, my soul, and my profession.
If you want your funds to go toward MoCo (and thus Firefox development), your best bet is to subscribe to Mozilla VPN or Pocket Premium.
[1] https://foundation.mozilla.org/en/initiatives/