This is one of my favourite add-ons for FIrefox! It's nice to be able to archive videos for my personal use when not connected to the Internet.

UPDATE (05/2021): After many years I still cannot say enough good things about this downloader! Wherever I find a video that I want to archive for future reference, I can use this plugin to do it! I know some people say it's "broken". Don't believe it! I've rarely had it fail, and when it has it is always because of issues with my configuration and not the fault of the software!