Description: Hover over a folder to open it and then you see list of things inside it, but let us focus on folders inside this folder. Those folders - when clicked on act as a link - when hovered they open list of things inside them. Also this applies to every folder (main folder too) and every nested folder. Additional improvement to this: Folder/bookmark that is hovered gets bigger (after some delay) and maybe different favicon if desired.

I think that is already possible with usage of greasemonkey and stylish. Anyone that know how to do this? Add-on for this (with gui to let user set things) would be awesome.