Go to /Users/[your username]/Library/Application Support/Firefox/Profiles/[*].default/chrome

Rename file userChrome-example.css to userChrome.css
Open it with text editor and copy and paste the following code after the line which starts with “@namespace”:

/* Hide Bookmark Folder Icon */
.bookmark-item[type="menu"] > .toolbarbutton-icon {
display: none !important;
}

/* Remove Bookmark Item Icons */
#personal-bookmarks toolbarbutton:not([type=menu]) image {
display: none !important;
-moz-margin-end: 0px !important;
}

Save it, restart Frirefox, all finsished.

Instructions slightly modified from the Firefox support forum