Append to your current userChrome.css file if it exists. This Firefox css file allows custom Windows navigation buttons.

userChrome.css

@namespace xul url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
 @namespace html url("http://www.w3.org/1999/xhtml");
 .titlebar-button {
     background-color: transparent !important;
     transition: background-color 0.5s ease;
     height: 0px;
}
 .titlebar-button>.toolbarbutton-icon {
     list-style-image: none;
}
 .titlebar-button:hover {
     background-color: rgba(121, 121, 121, 0.1) !important;
}
 #titlebar-close:hover {
     background-color: rgba(121, 121, 121, 0.1) !important;
}
 #titlebar-close:hover>.toolbarbutton-icon {
     list-style-image: url("chrome://browser/skin/caption-buttons.svg#close-white") !important;
}
 #main-window {
     -moz-appearance: -moz-win-glass !important;
     background: transparent !important;
}
 #navigator-toolbox {
     background: transparent !important 
}

background-color: rgba(121, 121, 121, 0.1) !important;

you can edit this line to change the overlay that shows when you hover