Firefox - Auto-hide Bookmark Bar | Works with Stylish

firefox-bm-ah.css

 @namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
 
 #PersonalToolbar {
    visibility: collapse !important;
    opacity: 0.01;
    -moz-transition: visibility 0.5s, opacity 0.5s ease !important;
    position: fixed !important;
    width: 100% !important;
    padding: 3px 3px 3px 3px !important;
    z-index: 10;
    top: 59px !important;  /*  59px OSX  /  71px Windows 8  - Default Theme */
}

#navigator-toolbox:hover #PersonalToolbar {
    visibility: visible !important;
    opacity: 1;   
}