Firefox Picture-in-Picture
PiP is such a great feature.  I have no idea how easy/hard it was to implement but it feels nice to use.  It does exactly what you'd expect it to do and nothing more.I use it while watching overwatch league games in the browser because it allows me to resize the popped-out video arbitrarily, instead of the site's  embedded youtube player options of either tiny-window-in-browser or full-screen sizes (there is no theater mode in the embedded player for some reason).