With every update, FF became slower and using more memory. I heavily use youtube, google search and reading news.
1) Youtube : When wieving any video/music, jumping forward stops and waits indefinitely - only a spinning circle is shown : until I scroll up/down at that youtube page; then the play resumes. This happens for last 3-4 years.
2) Scrolling page with mouse wheel waits 2-3 seconds, once in every 20 seconds (or so). This started lately (I'm not sure when it began).
3) We users become familiar with the look, colors, the width and height of the tabs, etc. with time. Please do not change the layout too much and too often.