The compact density option for the address bar is now gone, and not only that, the title bar is now a lot taller than before. The most substantial redesign, which is being prepared for the next release, called Proton, promises to drive most power users away because it's broken on a number of levels and makes using the browser a very unpleasant experience. Google Chrome and Apple Safari look almost the same as they did a decade ago. Seeing that, someone at Mozilla probably decided that the best way to entice people back is by changing its UI, thus Firefox has already seen quite a huge number of changes despite other major web browsers staying relatively the same in terms of their visuals i.e.

Tashkinov writes: Over the past ten years, Firefox market share has decreased substantially and the web browser has lost its appeal and coolness.