I just love the ponywear backcards! (Although for some reason Firefly and Baby Firefly are earth ponies on this one . . . also Firefly can't actually wear this set because of her attached leg.)
It's probably not worth super much because the card has quite a bit of shelfwear (looks kind of wrinkled and also the hanging tab at the top is gone. But if I'm reading the tweets correctly, her mother (who runs a vintage shop) found it at an auction "in the wild", which is always MUCH more exciting than buying off eBay or at a convention, and adds to the sentimental value.
(Plus even if it's not worth many $$$ it's just super cool to find MOC ponywear.)