Firefox 31 is the first browser to support CSS3 Variables,
so here are my first CSS hacks using them.

-Jeff

Example Usage:

This will show up in white on dark blue in Firefox >= 31

/* Firefox 31+ (CSS3 Variable Method - Underscore Hack) */

_:-moz-tree-row(hover), .selector 
{
   --myvar1: navy; background-color: var(--myvar1);
   --myvar2: white; color: var(--myvar2); 
}

/* Firefox 31+ (CSS3 Variable Method - Supports Hack) */

@supports (-moz-appearance:none) 
{
   .selector 
   {
      --myvar1: navy; background-color: var(--myvar1);
      --myvar2: white; color: var(--myvar2); 
   }
}