Hi mccfrank, there is a setting to make drop-down menus "stick open" until you press Esc if that would help:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste pop and pause while the list is filtered

(3) Double-click the ui.popup.disable_autohide preference to switch the value from true to false

What userChrome.css rules did you try?

Perhaps try something along these lines (example screenshot attached):

1. PlacesToolbarItems menupopup > hbox {

 color: blue !important;
 background: rgba(221, 238, 255, 0.3) none !important;

}

That might not work in future versions due to changes in how bookmarks lists are displayed, but let's cross that bridge when we come to it.

See also: https://www.userchrome.org/how-create-userchrome-css.html

Change Font Color

To change the font color:

   Click the menu button Fx57Menu and select Options.
   In the General panel, go down to Language and Appearance.
   Under Fonts and Colors, click the Colors… button.
   Clicking on any of the colored rectangles will show you the possible colors you can choose from. Select the color you want by clicking one of the colored rectangles.

   Fx38esrColors

   Click OK to close the Colors window.
   Close the about:preferences page. Any changes you've made will automatically be saved. 

Custom Font Color

Some websites have custom font colors, to disable them:

   Click the menu button Fx57Menu and select Options.
   In the General panel, go down to Language and Appearance.
   Under Fonts and Colors, click the Colors… button.
   In the window that opens, set Override the colors specified by the page with my selections above to Always.

   Fx47-pref-overridecolors-always

   Click OK to close the Colors window.
   Close the about:preferences page. Any changes you've made will automatically be saved. 

Other settings for Colors

Text and Background: Here you can change the default text and background color to be used on web pages that haven't specified that information. Click on the color samples to select colors.

   Use system colors: Check this option to use the colors defined in your operating system settings instead of the colors specified above.