How do you propose they finance development if we consider simple integrations with zero privacy or customization impact "selling out"?
They would need to finance a lot less development if they stopped changing things and just worked on maintaining a limited core functionality including a nice extension system. That's it. Most of their "development" over the last 10 years has been a total waste of time.
Can you really say with a straight face, that not doing vertical tabs or chat shortcut would allow them to operate on much smaller budget?
Either way, all I'm saying is when I look at the UI changes that have been made to Firefox over a period of years, I believe their value to me is closer to zero than their development cost is.