> the funding relationship between the Mozilla Foundation and Firefox isn't clear
It's not really an unknown. Donations to Mozilla Foundation do not and cannot fund Firefox development.
> if Google's funding were to become insufficient, then the Foundation would step in to fund it if they have the funds
There are two assumptions here that are off. One is that the Foundation could come up with the kind of money that the Corporation spends on Firefox. The other is that the Corporation is somehow in a tight situation financially. Reality is opposite to both.
First, the Corporation is not strapped for cash. It brings in a lot of money. Like, in-the-neighborhood-of-half-a-billion-dollars-a-year a lot. It spent something like 29MM acquiring Pocket (which is still as closed source now as it was 3 years ago), and it manages to spend 50MM on its marketing campaigns that most people aren't even aware exist. Under the current arrangement the Corporation funds the Foundation, paying a percentage of its revenues to allow the Corporation to trade on Mozilla's name. (If that sounds like a weird way to put it, consider revisiting your other assumptions about Mozilla and Firefox.)