I understand it's deprecated in FireFox, and its replacement is currentColor. My impression is that it's used for SVG-related stuff. But what the heck is it? :)
It's not really for SVG-related stuff.
It does the same as currentColor, e.g.:
a {
color:purple;
outline-color:-moz-use-text-color;
}
It was a mozilla specific CSS thing until it became standardized (CSS3)