Isn’t that what Google does when they pay for being the default? Real and honest question because I would imagine that the money from them would just as easily be used the same way you say donating to the Mozilla Foundation would be used. No money going to the foundation would also mean that the only way to fund the projects would be completely on relying on Google. Or at least that is how I understand it, and I am more than ready to be wrong. And if donating to the foundation means nothing. Then what about paying for the optional stuff like the VPN/Relay? Even if it goes to the foundation, it (at least per what they state and ideally does) claim to use funding to push into the politics of internet related projects. Which is also important for being able to fight legal battles like anti-trust (again ideally since I am not defaultly saying funds can’t and/or aren’t used incorrectly).
No money going to the foundation would also mean that the only way to fund the projects would be completely on relying on Google.
No. You could just directly donate to the developers (not to Firefox, there’s a difference). Though that requires the developers to take the firsts step: any web developer worth their salt knows how effed up the system is with donations going to corps, and will open up their own tip jar / donations box in a system like, dunno, Paypal or ko-fi.