"but what about all the other distros?"
They don't do most of the work. They mostly repackage someone else's work with some changes, and may work with limited resources. They don't build full products from the ground up.
Firefox is not a skin over WebKit or Edge, it's much more expensive to develop and market. Do you believe building a rendering engine is something you can do in your spare time?
Do you really believe every product can be developed the same way, especially with a bunch of unpaid amateurs in their free time? Do you understand what dedicated resources you need to develop a product like Firefox, and keep it updated?
Look, how Chrome became the main browser? Because it's developed by people in their free time, or because there's a Moloch like Google behind it?
Keep on believing in the FOSS Nirvana, it's just a way for company like Google to destroy competition and took full control of what you can use.
Biting the hand that feeds IT
