The problem is FF had once been that "1000% of all features you can imagine most of which are used by at most 1% or 2% percent of users" browser. This was so b

The problem is FF had once been that "1000% of all features you can imagine most of which are used by at most 1% or 2% percent of users" browser. This was so because of the huge customizability of XUl and other parts. But this became a maintenance nightmare the moment it was put in context with requirements like stability (there where many extensions which made FF unstable) and security (how extensions are sandboxed, how likely they can escape, what damage they can do without an sandbox escape), change (new internal engine, changed rendering pipeline etc.). A Company of Googles size could probably have maintained it, but for FF it became increasingly more infeasible to go on like that. So they switched to remove XUL and support mainly the parts most users use, sometimes adding new features but also removing them if they don't gain any traction in the user base. I personally see little use for the compact density as it just removes some spacing which is mostly vertical. So (at least in my case) using compact spacing doesn't at all create more space for tabs or similar it just adds a view more pixel to the vertical axis of the websites view. But then I'm only using 1080p and HiDPI screens. I guess it might matter if you are still on a 720p screen or alternatively have a high layout scaling (font size etc.) due to other reasons.