Regarding 1, although a single data point does not a trend make, I will say that I switched away from Firefox due to multiple UI changes and removing customization options. I switched to Pale Moon.
In other words, personally, you are incorrect. FF no longer "still offers the most customizable UI of any browser you've heard of". (Simple enough: FF removed options that PM kept. Hence, FF is not more customizable than PM.)
I switched away from Firefox for the same reasons. Even if the UI was as customizable as is claimed, I don't want to waste my time recustimizing it every six weeks, or whenever a new Firefox release comes out.
In other words, personally, you are incorrect. FF no longer "still offers the most customizable UI of any browser you've heard of". (Simple enough: FF removed options that PM kept. Hence, FF is not more customizable than PM.)