You are aware that there are full-time developers working at Mozilla, yes? Developing a browser is not a hobby-project that you can pull off with some volunteers in their free time. You need professionals that work on such a giant project with their full attention.
Developing Firefox is their job. And of course they want to get paid for that (and deserve it). Just like you get paid for your actual job.
How does someone deserve to be paid for work done? Is that your question?
Is this some kind of pathetic troll attempt?
I will not reward that with further attention.