I had that on my old computer for several years. It was very useful, but when Firefox upgraded to version 15, it was incompatible with my operating system and Flash version 11, so wouldn't let me see FLV content from any site. The only way I could view FLVs was to downgrade to Flash version 10, but then I couldn't play them back, as it was now an outdated driver.
I might have to convert them to play them now, so that's probably where the converter comes in... which I never got because I never needed it.

But if you are using Firefox, just go to the "Add ons" tab in the menu, and search for add ons... type in FLV downloader or flash downloader, and it will actually give you about a dozen programs that it supports to record videos from sites like youtube.