I use Firefox ext Video downloader to download videos of the internet but there is a problem: After I convert them via ffmpeg

ffmpeg -i Movie.flv -ar 22050 -ab 32 -f flv -s 320x240 Movie.avi

I play it with mplayer but I cannot fast forward or rewind to a certain point it appears that it is always at the end even though the movie just starts. The movie works fine but I can't get it to a certain point.