Instead of doing all the work below, you should first try installing this plug-in: Windows Media Player Firefox Plugin
Every once in awhile, I’ll hit a video on my gallery that requires me to download a codec, but telling it to download fails to find a codec that’ll work. If I copy and paste the URL into IE, the video plays fine. I started searching the Gallery forums.
I found this thread: Combination Gallery 2/AVI/Firefox does not display video, and it appears a lot of people are hitting this issue:
When adding an AVI file to my gallery it is recognized and ffmpeg creates a thumbnail for it. Viewing the item with Internet Explorer will result in the video being displayed.
However this does not work with Firefox. For some reason it seems that the Windows Media Player plugin in firefox does not recognize the video/x-msvideo MIME-type. Check about:plugins and scroll to Windows Media Player to confirm this.
Technically this is not a Gallery problem, but a bug by Microsoft in there Firefox support. However since Microsoft is not very likely to fix this it would be nice if someone could provide a work-around and integrate this into Gallery.
Before, I would just tell every video to use mime type: video/x-ms-wmv, however, Gallery2 supports a wider range of mime types, so I decided to let it do its own thing.
It turns out, to fix this problem, you need to associate video/x-msvideo in Firefox. Instructions provided by: Avi in firefox.
To do this, open the following file in notepad:
Search for “Windows Media Player Plug-in Dynamic Link Library” and add the bolded lines:
Windows Media Player Plug-in Dynamic Link Library|$
Increasing 9 to 10 tells it that there’s 10 mime types you should associate this plug-in with. Adding the video/x-msvideo line tells it to associate mime types of that with Windows Media Player. Afterwards, all my avi files began to play fine.
After saving this file, restart Firefox and video with the mime type of x-msvideo should play fine.