Mozilla Corporation is different from Mozilla Foundation. The Foundation runs off of donations and occasional cash infusions from the corporation. Basically, the Foundation's work doesn't affect the engineering budgets at all.

This is little more than 1 engineer's salary and benefits for a year.

Can you explain how this is Mozilla "throw[ing] its money away"?

If for that money one can get only one manyear in the bay area due to the cost of living, wouldn't it make much more sense to hire 4 people for a year elsewhere?

It seems obviously more complicated than that, otherwise the bay area wouldn't be what it is.

For companies that are physically there, but afaik Mozilla is quite spread out and people work remotely most of the time.

Mozilla has both multiple offices and remote employees.

So do various other companies with significant physical presences in the Bay Area (Google and Facebook come to mind).

I would guess it is closer to the cost of 1 engineer, maybe below. Conservative estimate of the average engineer at a company in the bay area is above 100K in cash compensation plus the cost multiplier of an employee is easily 2x once you account for benefits and other employer costs.