Bookmarks Toolbar   Blue/Folders, Red/Bookmarks 

///////////////////////
Bookmark folders shown without icons in BLUE,  and any bookmark not  in a folder would be considered  special and would be shown without icons  in RED.

The idea here is to use minimum real estate for  Bookmarks Toolbar,  so folders are used almost exclusively, and try to keep the folder names to 1-4  characters.  I know what my folders are so they don't need long names.    Most of  my keyword shortcuts are in a folder named "K" (use "K_"  for longer name)..

If the height can be adjusted properly then  it would  provide a much shorter and smaller  personal bookmarks bar.    In the meantime, I am leaving  out the change for  font-size which you  could set to 8px up to 14px, I  will be using 9px myself and have coded that into the sample.   

This is one of several styles to minimize size of toolbars, look for other styles under my name, user 12592, that  include "Minimal" in their style names.   Currently there is some overlap in toolbars,  since I've not learned how to reduce the size of the selection bubbles to be within the toolbars height.   The after shot should show about twice as many folders, and be about 1/2 height of the original.   

Updates:
2008-08-04 changed code to use 9px,  comment out or change if you want something else.
/////////////////////////////

/* the @namespace doesn't seem to matter for Windows but included */
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* Bookmarks Toolbar   Blue/Folders, Red/Bookmarks */
/* ================================================ */
/* David McRitchie,  2008-07-26, updated 2008-08-04 coded for 9px  */
/*   there should be a new after picture for 9px */

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

/*Changes to Bookmarks toolbar remove file/folder icons, change folder names to blue, and filenames to red.  Changes do not affect the drop-downs where you will still see the folder/file icons.  The bookmarks drop down width was increased. See these three topics in the userChrome.css code. */

/* 2008-08-03 fixed font-size but still have large toolbar height */
/* Make bookmark folders items bold (900) and navy blue (#0000EE)*/
/*  use 400  #0000E8 on main,  400 #800000 on firefox portable */
 .bookmark-item[type="menu"] > .toolbarbutton-text {  
   font-weight: 500 !important; color:#0000E8 !important;  }
/* make bookmark file items on toolbar red */ 
 .bookmark-item > .toolbarbutton-text {
  font-weight: 100 !important;   color: red !important;  
  font-size: 9px  !important;}  /** choose 8px up to 14 px */
  /* you can comment out font-size choice but I suggest  9px */

#PersonalToolbar {min-height:18px !important; 
  margin-top:-1px !important;  /* accept 2px bubble overlap */
  max-height:18px !important; }  /*font-size + 9px to include hangers */
 /* an improvement but still have oversized item selection bubble 
   would prefer -2px instead of -1px for top margin */

.bookmark-item > .toolbarbutton-text {
  -moz-appearance: none !important;
  background-image: none !important;
  padding: 0 2px 0 2px !important; 
  margin: -5px 0 0 0 !important;  }


/* change FONTSIZE on text with icons restricted to personal bookmarks*/
/* .bookmark-item > .toolbarbutton-text {-moz-appearance: none !important; */

/* remove folder/file icons from bookmarks toolbar  
 http://www.gfxoasis.com/board/lofiversion/index.php?t11762.html  */
.bookmark-item > .toolbarbutton-icon {display: none !important; } 

/* widen all bookmarks dropdown menus -- Chris Ilias 2007-01-12*/
menu.bookmark-item, menuitem.bookmark-item { max-width: 36em !important; }