I love using Firefox, but the UI has a number of bugs not least more than a few with drag and drop bookmarks and some have been there years!
For example try creating a folder in the bookmarks, then drag that new folder to a new place in the bookmarks, then try to drop the URL into that new folder. It won't appear. Why? ... well try this, open up a new window with CTRL+N, then look in that version of the bookmarks with this new folder and as if by magic the URL appears.
Here's another one, try dragging a URL into the bookmarks when a popup downloading complete message appears because if it does appear, you then won't be able to drop the URL into the bookmarks. It will be lost, even though you are still holding the bloody URL link icon!
Here's another one, try dragging a URL into the bookmarks with folders within folders. The bookmarks main folders get confused and close these windows.
Here's another one, try dragging a URL into the bookmarks with more links than fit in the window, the bloody folder of links scrolls and you loose your sub folder scrolling off screen, which you have to scroll back to.
Its such wonderful news the UI guy is going. Now someone can hopefully fix the bloody bugs in the UI!