When menus are too big to display, autorepeatbuttons (arrows) are added to the top and bottom of the menu. They scroll the menu when when hovered over, but not when something is dragged over them (https://bugzilla.mozilla.org/show_bug.cgi?id=194319). This extension works around that, until bug 194319 is fixed. It also prevents premature bookmarks menu hiding (bug 349932).
If you have any problems with this extension, please email me (my name @gmail.com). I do not check the comments on a regular basis, so you will get a much quicker response.
If you like my Firefox extensions, check out my (Greasemonkey) user scripts! (http://userscripts.org/people/336)