FLV is an Adobe Flash video file that's becoming less common, because most sites are moving to HTM5. That
said, very few players can play FLV files locally. Thankfully, you can adopt VLC Media Player or Windows Media Player to view FLV videos.
However for the latter, you need to install CCCP codec.
Pro Tip: If you are looking for a handy FLV to MP4 converter free download, don't omit our mighty media player, called Wondershare UniConverter - desktop vesion of Online UniConverter.