Can you post a picture?

Here's the drop down. Formerly, when I hovered over the right edge, a scroll bar appeared.

There is a scroll button showing at the bottom of that menu list.
Firefox doesn't show a scroll bar in the bookmarks menu by default, so if you had one then it was added by an extension.

You can open the Bookmarks in the sidebar to have a scroll bar.

• Toggle the Bookmarks sidebar on/off via "View > Sidebar > Bookmarks" or Ctrl+B (Command+B on Mac)
• Toggle the History sidebar on/off via "View > Sidebar > History" or Ctrl+H (Command+H on Mac)

That scroll bar at the bottom isn't very useful. I wonder what extension I had that allowed me to have a scroll bar on the side?

I think I found it... Bookmarks Scrollbar.... and not available for newest version of Firefox.

You can achieve this without an extension via code in userChrome.css

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

/* bookmarks: scroll bar */
#appmenu_bookmarks scrollbox,
#bookmarksMenuPopup scrollbox,
#bookmarks-menu-button scrollbox,
#PlacesToolbarItems scrollbox {overflow-y:auto!important}

#appmenu_bookmarks autorepeatbutton,
#bookmarksMenuPopup autorepeatbutton,
#bookmarks-menu-button autorepeatbutton,
#PlacesToolbarItems autorepeatbutton {display:none!important}

/* list all tabs: scroll bar */
#alltabs-popup scrollbox {overflow-y:auto!important}

#alltabs-popup .autorepeatbutton-up,
#alltabs-popup .autorepeatbutton-down {display:none!important}

Thank you! That worked!

You're welcome.

I'm using this already for a few years as it is much easier than those automatic working scroll buttons to move content into view like live feed items.

You can use the DOM Inspector to get the IDs of other drop-down list possibly added by extensions.