The :-moz-system-metric(scrollbar-thumb-proportional)CSSpseudo-class will match an element if the computer's user interface uses proportional scrollbar thumbs; that is, the draggable thumb on the scrollbar resizes to indicate the relative size of the visible area of the document.
The :-moz-system-metric(windows-default-theme) CSS pseudo-class matches an element if the user is currently using one of the following themes in Windows: Luna, Royale, Zune, or Aero (i.e., Vista Basic, Vista Standard, or Aero Glass). This will exclude Windows Classic themes as well as third-party themes.
The :-moz-ui-invalid CSS pseudo-class represents any validated form element whose value isn't valid based on their validation constraints, in certain circumstances. This pseudo-class is applied according to the following rules:
The negation CSS pseudo-class, :not(X), is a functional notation taking a simple selector X as an argument. It matches an element that is not represented by the argument. X must not contain another negation selector.
The :only-child CSS pseudo-class represents any element which is the only child of its parent. This is the same as :first-child:last-child or :nth-child(1):nth-last-child(1), but with a lower specificity.
The :optional CSS pseudo-class represents any <input> or <textarea> element that does not have the required attribute set on it. This allows forms to easily indicate optional fields, and to style them accordingly.
The :out-of-range CSS pseudo-class matches when an element has its value attribute outside the specified range limitations for this element. It allows the page to give a feedback that the value currently defined using the element is outside the range limits. A value can be outside of a range if it is either smaller or larger than maximum and minimum set values.
The :required CSS pseudo-class represents any <input> element that has the required attribute set on it. This allows forms to easily indicate which fields must have valid data before the form can be submitted.
The :root CSS pseudo-class matches the root element of a tree representing the document. Applied to HTML, :root represents the <html> element and is identical to the selector html, except that its specificity is higher.
The :visited CSS pseudo-class lets you select only links that have been visited. This style may be overridden by any other link-related pseudo-classes, that is :link, :hover, and :active, appearing in subsequent rules. In order to style appropriately links, you need to put the :visited rule after the :link rule but before the other ones, defined in the LVHA-order: :link — :visited — :hover — :active.
The :any-linkCSSpseudo-class represents an element that acts as the source anchor of a hyperlink independent of whether it has been visited, that is, it matches every <a>, <area> or <link> elements with an href attribute. So, it matches all elements that match :link or :visited.
The :-moz-locale-dir(rtl)CSSpseudo-class matches an element if the user interface is being displayed right-to-left. This is determined by the preference intl.uidirection.locale (where locale is the current locale) being set to "rtl".
The :-moz-placeholderpseudo-class represents any form element displaying placeholder text. This allows web developers and theme designers to customize the appearance of placeholder text, which is a light grey color by default. This may not work well if you've changed the background color of your form fields to be a similar color, for example, so you can use this pseudo-class to change the placeholder text color.