I've been using Firefox for about as long as a person could possibly have been using Firefox and I think this just happens every time they change the UI.
There is always a shock when a new design comes out. There was so much hate for the border less design youtube has right now but when I look back at older youtubes and older versions of almost anything, the new ones certainly feel better.
The only exception was the original iOS. They both hold up today. The original was a little cheesy but was still close enough to classy to work.
Just looks better and now we are used to it. Functionality is not better. We now have to think more about before clicking buttons and more load on brain.