#237 CSS :nth-last-child() not detected

[lonelypixel]

In the following code, the keyword named in the subject is not recognised. Instead, the blue colour ends after "nth-last". The rest remains black.

/* Remove "Open all in tabs" command from bookmarks toolbar dropdown menus */
toolbarbutton.bookmark-item menupopup[placespopup="true"] > menuseparator:nth-last-child(-n+2),
toolbarbutton.bookmark-item menupopup[placespopup="true"] > menuseparator:nth-last-child(-n+2) + menuitem
{
   display: none !important;
}