If only the Mozilla foundation would spend their money on Firefox only ...

I thought spending money on Firefox OS and rust projects was money well spent at the time. Wish it had continued

They don't spend donations on Firefox at all.

Do you have a source for this?

I was pretty sure they said it fairly plainly on their own site but what I could immediately find was more roundabout. You can see the things donations are used for here: https://foundation.mozilla.org

For a more straightforward statement, try this HN post relaying one user's correspondence with the Mozilla Foundation: https://news.ycombinator.com/item?id=20895624