maicol07: the code works but it seems that Firefox 61 is totally buggy. It sometimes does not load userChrome.css at all, sometimes works, sometimes does not show window at all. And the frame is corrupted (e.g. no borders, no resize...) even without any userChrome.css. Previous version has no problem. (you can also try it without that line @namespace ...)
Also you can try this code:
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
@namespace html url("http://www.w3.org/1999/xhtml");
#main-window {
background: none !important;
-moz-appearance: -moz-win-glass !important;
}
.titlebar-button {
background-color: transparent !important;
transition: background-color 0.2s ease;
height: 25px;
}
.titlebar-button > .toolbarbutton-icon {
list-style-image: none;
}
.titlebar-button:hover {
background-color: rgba(117, 167, 215, 0.6) !important;
}
#titlebar-close:hover {
background-color: rgba(255, 220, 220, 0.4) !important;
}
#titlebar-close:hover > .toolbarbutton-icon {
list-style-image: url("chrome://browser/skin/caption-buttons.svg#close-white") !important;
}