Out of the box, the Firefox add bookmark interface is very simple/limited (version 3.5 at least). As such, if you have a detailed folder breakdown you have to go through the drop list menu to get the interface where you can organize the new addition. If you do a lot of bookmarking this can be annoying, so to help with this check out the OpenBook add-on.
This extension allows for customization of the Add Bookmark dialog. The bookmarks tree can be automatically opened and elements can be revealed or hidden. Also, the dialog can be resized!
While the functionality of this add-on is straight-forward, to me it is much better than what Firefox has built in. Once you have started using it, you won’t go back.

