Like most people I was looking for a replacement for Save File To since it stopped working for me even in the legacy version of FF. This is almost but not quite it. I have many deeply nested directories and need to be able to route images to them quickly. Manually typing out everything that needs to go into the context menus isn't feasible. I was hoping that there would be a dynamic drilldown from the context menu itself, which would only require the user to specify the root once, but it doesn't work like that. If you specify C:\images, that's all you get. It doesn't show more submenus under that unless you go to the settings and laboriously add them all.
Going as far as you can and shift-clicking for a dialog then using the "recent" directories works ok, but this extension would be a life-changer if it let you choose subdirectories more dynamically.