Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Summary
The :-moz-locale-dir(rtl) CSS pseudo-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".
This allows extensions (and themes) to easily adapt their user interface based on the user's locale's needs. This can vary from window to window, and even from tab to tab. This also allows extensions to work even if they don't support the user's default locale, since they can support both left-to-right and right-to-left layouts without worrying about the locale specifics.
This selector does not work properly from HTML; it never matches, no matter whether the UI locale is left-to-right or right-to-left.