Supporting old api versions means the app is supported by more phones. International android versions can be quite old. While supporting old api versions may seem bad, it doesn't mean that Firefox cannot take advantage of new api features too. The code can simply check if the new features are available and use them.

Why do you care what the supported api version is?

When you have that much money, you fund every candidate. They may not win this round, but who knows what will happen next time.