I recently starting having an issue where my uploaded FLV files would not play. I had never had this problem before.

I found the problem was the uploaded filemime in MySQL was being set to "video/flv" which was not a valid playing mimetype for Flashvideo. If I manually set it to "application/octet-stream" in MySQL it would play just fine. Also, if I uploaded the file via Internet Explorer 8 it worked correctly (and it had a application/octet-stream filemime in MySQL after uploading). Using Firefox 3.6 would cause it to have a "video/flv" filemime in MySQL.

The way I fixed this so I could continue using Firefox was to change flashvideo.module line 21 to read:
define('FLASHVIDEO_MIMETYPE_STRING', "flv-application/octet-stream\napplication/octet-stream\napplication/x-flash-video\nvideo/x-flv\nvideo/flv");