I haven't tried these methods but I found 2 ways:
1) Edit your userChrome.css file.
Enable multiple rows of your current Bookmarks Toolbar:
The Bookmarks Toolbar is not limited to one row. It can display multiple rows by adding the following.
/* Multi-Row Bookmarks Toolbar */
#bookmarks-ptf {display:block}
#bookmarks-ptf toolbarseparator {display:inline}
I found this information
here.
2) Install the
Stylish extension first and then load this code:
@namespace url(
http://www.mozilla.org/keymaster/gat....is.only.xul);
/* Multi-row bookmarks toolbar */
#bookmarks-ptf {display:block !important; }
#bookmarks-ptf toolbarseparator {display:inline !important; }
#bookmarks-ptf .bookmark-item { visibility: visible !important;}
#overflow-padder { width: auto !important; }
#bookmarks-chevron { display: none !important; }
This code for Stylish can be added easily from
here.