Are you talking about the Bookmarks menu on the Menu Bar or about the menu of a folder item on the Bookmarks Toolbar?

#personal-bookmarks menupopup {
 color:#000 !important;
 background-color: #e8e8ff !important;
}

#bookmarksMenuPopup,
#bookmarksMenuPopup menupopup {
 color:#000 !important;
 background-color: #e8e8ff !important;
}

The menu of a folder item on the bookmarks toolbar, as in the dropdown menu you get when you click on a folder in the bookmarks toolbar. I tried both of those in the userChrome and it didn't change the back ground color.

Are you using the default theme (Tools > Add-ons > Themes) or a custom theme?

The #personal-bookmarks code should work for the Bookmarks Toolbar.

Yes, I disabled everything and switched to the default theme and it still did not work.

That didn't work but I used this code and it worked, not sure why since it is basically the same thing:

1. bookmarksMenuPopup menupopup arrowscrollbox,
2. personal-bookmarks menupopup arrowscrollbox {

background-color:#ffc !important; }

Does other code that you place in userChrome.css work ?

Make sure that the file has a @namespace line at the begin of the userChrome.css file.

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

See also http://kb.mozillazine.org/Editing_configuration#How_to_edit_configuration_files

How do I change ALL the drop-down menus' background color (File, Edit, View, etc.)?

Code like this should work for the menu bar:

#main-menubar menupopup {
 color:#000 !important;
 background-color: #e8e8ff !important;
}

cor-el, thank you! Your code worked, but you have to add arrowscrollbox after menupopup.

1. main-menubar menupopup arrowscrollbox {

color:#000 !important;
background-color: #e8e8ff !important;

}

I hadn't checked the CSS code, but I see (-moz-appearance: menupopup;) for the menupopup, so that is the most likely cause that you need to specifiy the next (child) element.

Adding -moz-appearance: none!important; should make it work in such cases.