One thing I love about Firefox is the Video DownloadHelper. Not only can it be used to download YouTube videos but it can also detect video in any webpage and offer a download link. This is good for situations where you let a YouTube video load only to find that it has stopped loading half way or something messed up. Also having a copy of the file lets you play it while you’re offline too. One thing that has always got me confused was the download links you get on a YouTube page as shown below

There are 2 .flv links and 2 .mp4 links and each file means something different. If you check YouTube’s wikipedia page there are 6 formats in use and as you can see the a high fmt value (which is the number after “HQ” in the download links above) is not necessarily the best quality and also an .mp4 file link is also not necessary the better quality file.

Confusing isn’t it? So I downloaded fmt 34,18 and 35 and the following are screen captures from the video of each of them. I did not download 720p or 1080p as I believe those 2 have the highest quality and the 1080p video link did not display in the listing too.

MP4 (fmt 18), resolution: 480×270:

MP4 (fmt 18) blow up to the same resolution (854×480) for comparison:

FLV (fmt 34), resolution 854×480 (am not sure why the resolution is the same as fmt 35 even though the specifications says it shouldn’t be):

FLV (fmt 35), resolution 854×480:

Based on this video it seems that MP4 makes the video blurry and soft while the FLV files (both format 34 and 35) seem hard to differentiate between. Also take note the file sizes:

• MP4 (fmt 18): 14.5MB
• FLV (fmt 34): 21.8MB
• FLV (fmt 35): 30.1MB
• MP4 (fmt 22): 66.8MB

Taking that into account, I recommending downloaded the MP4 if you are more concerned about file size (and data charges if using 3G to download) while stick with FLV 35 for good quality without huge file sizes of a 720p/1080p MP4 high definition video file.