Consider this scenario when browsing bookmark folders on the bookmarks toolbar (all root-level items): There are two folders, "Folder A" and "Folder B". When clicking "Folder A" to display the popup with the contents, and then wishing to switch to "Folder B" (because you're not sure where you put the bookmark, or you clicked the wrong folder by mistake) you first have to click outside the popup for "Folder A" to dismiss it and then click "Folder B" to open that popup.

In browsers like Mozilla Firefox, Google Chrome and Apple Safari, just hovering "Folder B" while the popup for "Folder A" is open will close the popup for "Folder A" and open the popup for "Folder B".

This would make using the bookmarks toolbar faster and less tedious.