This worked for me on some sites, but I noticed it was using a lot of CPU even when I wasn't downloading things (thank you, Firefox Task Manager).

Turns out it the "privacy" policy is wide open. Most companies won't sell your personally identifiable information, but exten.inc does: "We may share some or all of your personal information with our subsidiaries, joint ventures or other companies."

And it has full access to everything you do.

You should try Video Download Helper. It will download almost anything, and collects no information.