This is a switch bait scheme. You won't be able to download in proper HD formats unless you cough up some cash first. They explain how you "need powerful server infrastructure" to actually download in HD and how it's "limited to PRO users only". There are literally TONS of other Firefox addons that do the same job better, and for free. My suggestion to the developers is to find a real job. Makes you wonder why Mozilla is recommending this addon.