This is pretty much just an small rewrite of the 'css-lastchild.js' feature test already present on Modernizr to allow for the detection of browser support for the :first-child pseudo-selector. I am pretty sure that every browser that supports :last-child also supports :first-child (and vice-versa), and as such including this may sound a bit redundant, but if I am using :first-child anywhere I'd rather write

.firstchild {
    /* (...) */
}
.no-firstchild {
    /* (...) */
}

than .lastchild and .no-lastchild.