Here's the example pulled from their blog article. The text-shadow mixin is going to need to be updated to support this, or it would be nice if it was.
text-shadow: #99FFCC 0px 0px 10px; /* for browsers without spread distance */
text-shadow: #99FFCC 0px 0px 10px 10px; /* for browsers with the full spec */
Ideally the text-shadow mixin would just drop the spread parameter for the unsupported browsers.
@include text-shadow( #99FFCC 0px 0px 10px 10px );
Firefox has been considering adding this for over a year. https://bugzilla.mozilla.org/show_bug.cgi?id=655590
Submitting a patch for this with #877
Handle the new spread parameter for text-shadows. Closes GH-870.