phd-userChrome-multiline-bookmarks.css

/* multiline bookmarks */
toolbar#PersonalToolbar {
    margin-top: 3px !important;
    max-height: 1000000px !important;
}
#PlacesToolbar > hbox {
    display: block !important;
}
#PlacesToolbar #PlacesToolbarItems {
    display: flex;
    flex-wrap: wrap;
    overflow-x: visible !important;
    overflow-y: visible !important;
    contain: content !important;
    margin-top: -2px !important;
    margin-bottom: 1px !important;
}
#PlacesToolbar #PlacesToolbarItems toolbarbutton.bookmark-item {
    vertical-align: middle !important;
}
#PlacesToolbar #PlacesToolbarItems toolbarbutton.bookmark-item > label {
    margin-top: -1px !important;
    margin-bottom: -1px !important;
}
#PlacesToolbar #PlacesToolbarItems toolbarseparator {
    margin-bottom: -100px !important;
    max-width: 3px !important;
    padding: 0 0 0 1px !important;
}
#PlacesToolbar #PlacesToolbarItems toolbarbutton.bookmark-item > label::before {
    margin:auto;
}

/* multiline bookmarks: auto hide */
/*
#PlacesToolbar #PlacesToolbarItems {
    max-height: 0 !important;
}
#PlacesToolbar #PlacesToolbarItems:hover {
    max-height: none !important;
}
*/

/* hide first item to work-around bookmarks limit:
 * - https://bugzilla.mozilla.org/show_bug.cgi?id=1752735
 * - https://searchfox.org/mozilla-central/rev/78963fe42f8d5f582f84da84a5e78377b6c1fc32/browser/components/places/content/browserPlacesViews.js#1059-1065
 */
#PlacesToolbar #PlacesToolbarItems .bookmark-item:first-child {
    display: none;
}