It's losing because Firefox is now designers-driven product, not engineers-driven. Everything rots to the end when you give designers power over engineers.
This sounds like a false dichotomy but quite makes sense when it comes to browser market I believe. FF 'feels like' lagging behind for majority of users because of the hiccups here and there.
You do realize that without designers and tons of other people, there is no one to do the actual user research, UI, UX or general design work? Who would do this work?
A browser is to 99% a canvas for other software to draw on, and neither Safari, nor Edge, nor Chrome are investing terribly much into constant UX changes, so where does Mozilla's perceived "need" for UX changes come from?