I use a Firefox add-on called DownloadHelper which not only downloads streaming video (be it flv, avi, whatever), but will convert flvs to a format of your choice. That said, it's a bit awkward - once the file finishes downloading, it starts converting automatically at the cost of most of your system resources, and once finished the end quality isn't great.
But then, flv quality isn't great to begin with, hence its smaller bandwidth requirements. I'd concur with the VLC Player recommendations so you can just watch the flv without conversion.