Stay-Open Menu 3.1.0
le custom.firefox.lady
Facilitates opening multiple items from the Bookmarks Menu and various other bookmark/history related dropdown menus.
Maidir leis an mbreiseán seo
For extra convenience, turn on the auto-closing option, and the menu will automatically close after it detects that you've moved the mouse off the menu.
Known issue:
The Smart Location Bar Option has no effect on Seamonkey, i.e. the dropdown does not stay open due to different code used by SM (likely WONTFIX)