Custom Folder Icon for Bookmark Toolbar in Firefox, Iceweasel, PaleMoon...

What if your Mozilla-browser's favourite theme lacks nice folder icons?
You don't have to rewrite the theme, it's enough to create or edit your
profile-folder/chrome/userChrome.css e.g. like this:

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
/************************ DO NOT remove the Above line  **********************/
.bookmark-item[container="true"] {
list-style-image: url("file://<full-path-to-22px-png-image>") !important;
-moz-image-region: rect(0px 22px 22px 0px) !important;

Remember that the folder path still starts with a slash '/', so you end up having three slashes: 'file:///home/...'

...and of course restart your browser.