Without even scrolling down, there's a header describing the work they're doing for "Continuous work on speed, performance and compatibility", exactly what you asked for.

Anytime Firefox works on UI stuff, people seem to get really annoyed they aren't working on something else. You mention that "Reorganizing menus or adding alt text to PDFs won't make any impact on user adoption", I think it's the exact opposite. There is no greater thing the average everyday Joe cares more about than the UI, and if they just called it good 10 years ago, the project would be in an even worse state.