Amazon offers Prime members Firefly: The Complete Series on Blu-ray
with free shipping
. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $4.98.) That's tied with our mention from last May and the lowest total price we could find now by $4. This 3-disc set includes all 14 episodes, a gag reel, behind-the-scenes featurettes, deleted scenes, and audio commentary for select episodes.
Note that it is temporarily out of stock, but you can still order it at the above price.