They wouldn’t. The Corporation is a separate entity, I believe for tax reasons, allowing them to hold more money. I don’t think that it’s allowed to use it as a loophole to avoid regulations that apply to foundations, while still using that money to fund Foundation projects.
AFAIK the only way money flows from the Corporation to the Foundation is by the Co paying royalties to the Foundation for the use of the Firefox trademark. Obviously exactly how that number is determined is a little fuzzy, but I don’t think it (legally) can be just any number - it has to be justified somewhat. In any case, the Corporation is not short of money, so if the Foundation wanted more money to flow from it to the Foundation, a shortage of money due to CEO pay is not the reason.
(You are definitely right in the sense that Co money could be used to fund more Co projects. Those are not the same initiatives that would be funded by donations to the Foundation though, as money doesn’t flow from it to the Co. Think Common Voice, MozFest, lobbying, Privacy Not Included…)