I almost gave it 4 stars because it occasionally can't download a particular video. But heck, nothing is perfect, so 5 stars for all the hassle it has saved me-- no more hunting through source code, or de-obfuscating links to videos. And I appreciate the choice of formats. Smooth and seamless-- good job, guys.
(Edit: it's now been working for me for more than two years, which means the developer is staying on top of Firefox updates. A solid five stars still.)